I would love to see some custom behaviors too.
Booze Ravenous:
Like for example, you get a forgotten beast that is not initially hostile. He will break down your doors but is not interested in attacking dwarves per say. When it announced he is an extremely quick foe, that your dwarves will have a hard time attacking. But the FB has a weird behavior. His ONLY goal is to drink all your booze! He will break in to your fort, and scare the shit out of your dwarves by preceding directly to you alcohol stash and raid every last drop it can find. Left long enough, he will work his way through your whole stash, but there is a catch!
The more he drinks, the weaker he gets. In the beginning he has a ridiculously high dodge, but after about 200 drinks, it is dropped to half that, and your dwarves might be able to pick him off then. Of course if they try to kill him to soon, the FB will be hostile all the time, and it will slow the speed at which he drinks, and he will kill whatever dwarf wonders by.
Food Ravenous:
Same thing for with food.
Egg Laying:
A FB that will wonder into your base and lay eggs of badies all over the place. His presence is announced in the beginning, but after that he is invisible unless spotted, and so are the eggs.
Doppelganger:
Two ways this could go down. The doppelganger comings in with a migration wave, or it just pops in one day like an ambush. The Doppelgangers will disguise himself as a dwarve, and your will have never seen or heard of him. His goal is to steal artifacts (or the most expensive things you own), and he does not really desire to kill dwarves unless they are carrying the thing he wants. You will instantly be able to notice he is not right, when you try to V him, but most people will never notice.
He might even have to goal of sowing discontent in your base (presumably being paid a great deal of money to do so by someone who hates you). He might kill the mayer's pets, sneakily to make him miserable, and he might say really mean things to dwarves who are already not in a good mood. He might even lock doors from time to time.
Poltergeist:
This is like the normal bad spirits in the game but far far worse. More like peeves from harry potter. He can go through your walls, and is not effected by traps, and spends most of his time throwing things about. He might even posses someone every once in while, and make them start a fight. One way you might be able kill him is by ringing him with a filled stockpile of the item he hates (such as plimphelment seeds for instince), which will causes him to become miscible, and disappear for good.
Madness:
A FB that can make dwarves go made for a set time period, and run away. This is horrible for some military because a bunch of them might run off while you are trying to get them to fight. Dwarves that get caught with madness might have other benefits after the fact, like greater mental toughness. Madness monsters could have minions with them, that just reek haviock while your dwarves are trying to figure out what to do.
Water monster:
A monster that can summon, or exude water. This might making the ability to swim MUCH more important. The monster will walk around, fulling up stuff with water, and if he gets into your base, he might totally fill rooms with water that he is fighting in. He could even use jets of water as a weapon to knock dwarves back.