6-7: An 8 year old human child.
8-9: A small dog, a badger or deer.
10-11: Your average human
12-13: A large dog, or a human who routinely works heavy manual labor
14-15: A gnoll, a dire (small) animal
16-17: A bodybuilding human, a shark, a leopard
18-19: A minotaur. An extremeist bodybuilding human.
20-21: An ogre, a super-human.
6-7: A person with bad arthritis
8-9: An 8 year old human child
10-11: Your average human
12-13: A football player. A bugbear.
14-15: A skilled acrobat. A displacer beast.
16-17: An octopus. A master locksmith.
18-19: A freaky asian contortionist.
20-21: Gumby. An angel.
6-7: A frail old person.
8-9: A swarm of locusts. A cow.
10-11: Your average human
12-13: A rust monster. Someone who eat's healthy and takes vitamins.
14-15: A horse or troglydite.
16-17: A tiger, an assassin vine.
18-19: Bears, elephants
6-7: A troll. A man with learning difficulties. (7 and below is a speech problem)
8-9: A gnoll, a man who is just a touch slow but gets by. (small words and careful meaning, but not slurred or stupid)
10-11: Your average human.
12-13: Your average well-educated human. A lamia. A cloud giant.
14-15: A wraith. A will-o-wisp. A walking thesaurus.
16-17: A succubus, a beholder.
18-19: A mind flayer.
20-21: A titan. A human genious.
6-7: A human teenager. A githyanki.
8-9: A troll.
10-11: Your average human
12-13: Hyena, an Owlbear.
14-15: A sagely human, a philosopher, a psychiatrist, an owl.
16-17: A Lillend, an androsphynx.
18-19: A buddhist monk.
20-21: A unicorn, a storm giant.
6-7: A badger, bear, troll.
8-9: Gnoll, an ugly person.
10-11: Your Average Human
12-13: A Treant, a roper, a used car salesman.
14-15: A medusa, a storm giant, a priest.
16-17: An ogre mage, a pixie, a harpy, someone who is a professional speaker.
18-19: Nixies. Mr Suave. Fabio. Dick Clark.
20-21: Astral Deva.
Ability scores explained in detail
A character with low Strength lacks muscle mass. He receives penalties to hit in melee combat and penalties to damage with most weapons. He can't climb well, lift much weight or exert much force. While this might mean a character without athletic training, there are other options.
Weight factors into Strength - the larger you are, the greater your potential. For Moderately low scores (6 to 9) you might be large without muscle tone: you can throw that weight around, but you are weak for your size. For lower scores you might be emaciated: an apparition with sunken cheeks and fleshless limbs.
Age is another explanation. A low-Strength character could be old and frail, which works well for spellcasters, as age gives appearance of experience and knowledge. Perhaps a character is particularly young. Playing a youngster has it's advantages: adults ignore or underestimate childrens, and a child could be more flexible and easily hidden, but you should avoid life skills such as Knowledge, Profession or Craft. Regardless, experience translates into maturity, and with maturity could come Strength.
You might have personality quirks centered around your meager muscles. Since you can't participate in physical labor, you might see yourself about it, assuming every powerful frame holds a weak mind. You might take a "those who can't do, teach" attitude and direct the labor of others, or you might be envious of or deferential to those stronger than yourself. Even this most physical of abilities offers plenty of opportunities for roleplaying.
How To Compensate
For fighting, low-Strength characters should buy crossbows - the only weapons that don't rely on Strength for either attacking or damaging opponents. You can do a hefty amount of damage with them, and with ranged weapon feats you become a serious threat.
If you can't afford a crossbow, try carrying around the largest weapon available. You might look like Matthew Broderick holding Rutger Hauer's bastard sword in Ladyhawke, but when you roll 1d10 for damage you might at least do some harm if you hit.
Keep an eye on the party's back, you can prevent surprises and foul enemy sneak attacks.
With a good AC and decent hit points you can defend archers and spellcasters, or you can play shield man to someone with a weapon with reach. You might not directly damage opponents, but giving the sorcerer time to finish a spell can be critical in a dangerous fight.
Even yelling for the town guard or running around for help could turn the tide of a battle without the need for an attack roll.
A low-Dexterity character suffers penalties for ranged attacks, Armor Class, Reflex saves and any skills requiring coordination or agility. You're useless at moving quietly, can't maintain balance, neither throw nor catch well and routinely dodge right into attacks. "She's clumsy" is the first response to a low Dexterity, but several other possibilities exist.
A handicap might be the cause of a low Dexterity, such as the classic peg-leg of a pirate. Perhaps you recently lost an eye: the loss of depth perception and peripheral vision could explain a low Dexterity. Equilibrium-altering wounds could be the reason. Perhaps your wizard was too need the origin of a Shout spell, or maybe your priest heard her god's true voice.
Alternatively, you could have tremors and poor motor control: you could be a wizard whose studies of the unknown left her with twitches she can neither stop not explain, or a veteran whose constant drinking has dulled the memory of was but leaves her unsteady. Subterranean races often risk cave-ins, and many walk away with injured limbs and frayed nerves. Such people might be clumsy, but they are hardly inept, and their conditions can at least lend them a haunting history.
Roleplaying a low Dexterity depends on the score's rationale. Those hindered by a physical problem might use it to avoid hard or dangerous labor, or they might refuse assistance to prove they're as good as anyone else. You might be perpetually apologetic or constantly upset over your blunders. Alternatively, you might make jokes about your apparent ineptitude or develop excuses to cover for it ("Somebody keeps moving that plant!"). The key to roleplaying a low Dexterity lies in how the character feels about her fumbling and how she expresses it.
How to Compensate:
You'll have to accept that you won't hit at range, forcing you into close combat. Once in melee, you're going to be easy to hit. Low-Dexterity characters should buy the best armor available, regardless of armor penalties. If you already have a Dexterity modifier of -4, armor penalties are just reinforcing what we already know: you're no good at Dexterity-based skills. Even wizards and sorcerers should consider leather armor, since a 10% spell failure is a reasonable trade for getting their AC closer to double digits.
If you have a reach weapon you can use other party members as cover, standing in the second rank and attacking over their shoulders. The opponent faces to foes and your poor AC doesn't become an issue.
You can reduce some of the penalties of a low Dexterity score with feats (Improved Initiative and Lighting Reflexes), but his means sacrificing an advantage to offset a weakness. This is usually poor investment. Feats are better spent making you heroic, not raising you to average.
One fear you should consider taking with a low Dexterity is Expertise. If your Intelligence is high enough, pick this up - it lets you improve your AC when needed and allows you to take other feats that offset a poor AC. Few people attack well when disarmed or prone.
Low-Constitution characters have penalties on hit points and Fortitude saves, as well as Concentration checks when casting spells. They are easily knocked out and easily killed, since having poor hitpoints is a fatal flaw for an adventurer.
Many of the physical descriptions of a low Strength or Dexterity can translate well for a low Constitution. Characters who are extremely underweight or overweight might be easily winded or unable to resist injury. An eldery character's health could be ravaged by age. You could have old wounds (such as an arrow through the lung) or be a plague survivor. Old wounds explain a lack of stamina while emphasizing your will to survive.
More dramatically, you might be the victim of a long-term disease. For example, tuberculosis can take years to kill, leaving victims weak and coughing up blood, but Doc Holiday was the most feared gunman in the West even as tuberculosis was killing him. In fantasy settings, even more insidious illnesses exist, sapping your life while allowing limited activity. A character facing his own death head on and refusing to wait for it engenders respect.
A low Constitution often shows itself in personal behaviors. You might constantly push yourself too far and then collapse until forced to move again, or lean against the wall and rest at every opportunity. You might have remedies or medicines you constantly prepare to bolster your Strength, or obstinately ignore your problems. Roleplaying a low Constitution relies more on what you're doing and how you're doing it than what you say.
How to Compensate:
More than other characters, low-Constitution adventurers should avoid combat. Unless you're playing a class with large Hid Dice, a 3 Constitution leaves so few hit points that any attack equals death. But classes with a large Hit Dice are expected to handle more combat, however, putting you back at risk.
You must avoid taking damage, and that means wearing whatever gives you the best AC, following all the advice for finding cover and relying on guile whenever possible.
Fortunately, low Constitution penalizes neither ranged nor melee combat, so you can attack from range or defend up close as long as you avoid attacks. This is where skills and feats come in. Taking ranks in Tumble skill and gaining feats like Spring Attack can negate your enemies' ability to attack you.
Watch out for poison and creatures that cause Constitution damage, If you drop to 0 Constitution you die, and with a low Constitution score there's a good chance that the initial Constitution damage will kill you. Try to get your hands on Neutralize poison and Lesser Restoration potions. If you can, cast Neutralize Poison on traps and creatures before you come in contact with them.
Cunning could make combat unnecessary. Scatter coins among opponents to distract them; heave a spare lantern into the enemy; cut the rope holding a chandelier overhead. These certainly change the field of battle without calling down attacks.
Some feats help with a low Constitution (Great Fortitude, toughness, Combat Casting), but the only required feat is Toughness: the hit point bonus is a godsend for a low-Constitution character. You'll still have to worry about low hit points at higher levels, but at least you'll survive to see them.
A character with a low Intelligence learns slowly and reasons poorly. She has penalties on the number of skill points per level and on any skills based on learning.
Some thought should be applied to why you have such a low Intelligence. You might have never had an opportunity to study, or perhaps you were forcibly banned from doing so. You could've grown up on the streets, among war refugees, or as a slave, where any attempt at learning was severely punished. You might have been raised by animals, a fantasy classic for characters with a nature bent. While brilliant people might come from such backgrounds, most survive by intuition, guile, perception, and will - in other words, traits associated with Wisdom and Charisma, not Intelligence.
For a great challenge, you might be an amnesiac, forgetting anything more than 24 hours old. Head trauma occasionally causes such problems. You are reborn every morning, with no guide save those around you and whatever notes you left for yourself. You may need solid roleplaying skills before trying something this extreme.
When playing a low-Intelligence character you could play it for laughs, as with the barbarian who claims "newly discovered" towns and names them after her father. Or you could play it for terror by taking offense at people talking down to you and snapping into vindictive rages. You might insist on having a say in any discussions and want your not so helpful ideas respected, or you might keep your mouth shut to avoid looking the fool. You could repeat others' suggestions as your own or constantly request more time to work through problems. Remember that a character with a low Intelligence can learn and reason through everyday things, they simply can't do it quickly or reliably. You needn't stifle your every creative thought, as long as you aren't constantly brilliant or quick.
How to Compensate:
The mechanical challenge for a low Intelligence is your diminished skill points. This penalty usually wipes out all but the minimum 1 point per level. Playing a human restores one of those points, but you should rely on the special abilities of your class and one or two non-Intelligence-based class skills. Denied breadth of skills, you should focus on a few strong points.
Someone with a low Wisdom lacks common sense, perception, intuition and willpower. He is penalized on both his will saves and skills involving untrained experience or awareness.
A poor Wisdom score is the hardest to explain, since it covers such a broad area. Wisdom is something one gains with age, so you might consider a young character to explain a low Wisdom. Take care with this, since a well-played low-Wisdom child will quickly get himself into trouble and might drag the party down with him. Perhaps you come from a culture vastly different than the local norm: detail this culture with your DM, with both good and bad points, and stick with it. You'll miss local cultural cues, make assumptions based on false premises, and argue against things the rest of the party takes for granted ("Letting women drink ale instead of wine ? It will corrupt their souls!"). You wouldn't be stupid, merely ignorant of the local custom.
When playing a low-Wisdom character, make it clear that the party can't rely on you in the long term, In the short term, during the course of a fight or while exploring a dungeon, they can keep an eye on you and trust you will do your part. But if left on watch alone you might fall asleep, get engrossed in spell research, or otherwise lose focus. In delicate negotiations you'll let secrets slip or take offense where wiser diplomats would not. You might forget to mention things you've learned until a critical moment, or assume that your companions must know things, and thus never mention them. The party should probably put another PC to watch over you, just to make sure your impulsiveness doesn't endanger them.
How to Compensate
A low Wisdom offers comparatively few mechanical problems. Some important skills are reduced, and the penalty on Will saves can be troublesome, but all told the low-Wisdom character suffers few survival challenges. The feats of Alertness and Iron Will compensate for most hindrances, but at the usual cost of not honing your other strengths.
A low Charisma comes from a lack of faith in yourself, translating directly into an inability to interact with people. The character is penalized on all skills that involve social interaction, as well as any non-skill interaction checks.
When dealing with a low Charisma you should remember that these personality traits don't appear without cause - no one is born with a 3 Charisma. What caused you to lose faith in yourself ? Possibly your family or culture valued everything you were poor at and never praised your skills: shy to an extreme, you avoid conversation for fear of being insulted, belittle your abilities, and view any comment as criticism. You might be an escaped slave: deeply ashamed of your past, you either lash out verbally or meekly follow suggestions as orders. You might have once had personal faith, now shattered by some terrible event. This trauma need not have been your fault - you believe if to be so even if the world holds you blameless. An accidental killing, a failure of nerve on the battlefield, the inability to save a loved one, or failing a test of character can shake your self respect. Once that is lost, it is hard to gain the respect of others.
Alternatively, you might be extremely ugly. Not just homely, but malformed to the point where children scream and woman faint. faced with such reactions you see yourself as an outcast while still desperately wanting to be accepted by society. Note that this is very different from the high-Charisma half-orc who looks like ten miles of bad road but whose magnetism unites an army - appearance matters to Charisma only if you let it matter.
When roleplaying a poor-Charisma character, keep in mind why the character has such a low score and run with it. This might mean never voicing a suggestion (even if you know what to do), or voicing every single suggestion loudly and authoritatively - since if it isn't accepted it damages your already fragile sense of self. Belittle NPCs to prove you're better than them, and even make jibes at other PCs' expense (though not too often, or you'll reduce other people's enjoyment of the game). Here's another idea: start whispering ideas in character to another player, letting him voice them to the group - since your character thinks no one listens to her ideas, she can contribute without putting her ego at risk.
How to Compensate
If your character has a low Charisma, accept the fact that you aren't the party's spokesperson. Any interaction is going to suffer seriously, but you might pick one of the Charisma-based skills as focus for skill points, giving you a way to interact with a skill bonus.