A lot of mods do things like this.
Giving animals the LARGE_PREDATOR tag will cause them to randomly attack wandering adventurers. A high vision arc or EXTRAVISION will also help ensure that they spot the adventurer upon entering battle. Giving them high BRAVERY will make them more likely to attack threatening adventurers, and natural DISCIPLINE skill will keep them from running away. Making a bunch of creatures like this will make wandering the wilds significantly more RPG-ish with regular random encounters.
To make things even more interesting, you can divide individual 'monster' species into castes that don't resemble each other at all, which will allow groups of them to attack together. Like make one caste fast and melee-focused, one caste with a bunch of syndrome-inflicting reactions, one slow and durable caste... then they will attack as a team, forcing you to use strategy in battle.
Just make sure the monsters aren't too tough, since they will attack adventurers (not to mention fortresses) of all levels, and you don't want to be constantly in danger of being murdered by magical super-wolves. Or at least make them slow enough to run away from. Alternatively, you can divide them up by biome - low level adventurers should stay out of the swamps and deserts, for example. Or just restrict particularly dangerous monsters to EVIL biomes.
Adding extra butcher objects will allow you to give special rewards to monster hunters. You can then give adventurers and fortresses reactions that utilize these extra objects by collecting enough of them.