Some useful tags to consider:
BENIGN: Creatures with this tag will not scare dwarves by default and will neither attack nor run from them. If you build outside, they can walk right up to your citizens without problems. Good for large herbivores.
LARGE_PREDATOR: Will attack creatures smaller than them (but will usually flee from those larger). Will ambush travelers regardless of size.
AMBUSHPREDATOR: Will generally only attack if the target is a few spaces away. Less dangerous in the open because dwarves will usually avoid getting too close, but dangerous in places with narrow twisting corridors.
PRONE_TO_RAGE:# : Have a percent chance of flipping out and attacking everything (except for its allies) in sight. Even with a 1% chance, these creatures are dangerous. BENIGN only makes them more dangerous, since dwarves won't avoid them until it's too late. On the other hand, make excellent war animals.
VIEWRANGE:# : An underrated tag that is very handy for controlling behavior. It controls the distance at which this creature will react to other creatures. May make a creature much more or less dangerous, depending on other details.
HUNTS_VERMIN makes a creature chase and kill vermin, while GOBBLE_VERMIN_CLASS will actually eat vermin it encounters instead of leaving their remains around. Combine HUNTS_VERMIN with GOBBLE_VERMIN_CLASS: GENERAL_POISON for a creature that hunts and eats vermin.