I found that sometimes enemies can attack each other, or they attack animals and provoke their aggro on them.

Does anyone have know the patterns? Is their agressivenes based on kind (kith, beast, vessel...) or the behavior is more complex?

I don't know specifics but usually kith enemies will battle beats/wilder/primordial enemies as long as if they are not in the same group. 

You can nearly always lead trolls, boars etc. to an enemy kith group and they will fight. But it also works with vessels vs. beasts and so on (don't know if always, but often enough). 

I suspect there's some parameter which determines which enemies react hostile to each other. But I didn't look into it. 

I can say that the Kith x all part is true. But the beast x vessel i don't know. In Anslog Compass, the guls don't attack the xaurips and in Magran's Fork the will o' wisp will not attack the wolfs.

Life would be easier for solo players if PoE races had more hostility between them :(!

Right. But iirc the fampyrs & darguls in Russetwood will fight beetles for example. Maybe being rational makes a difference? Also the fampyrs start neutral and only turn hostile after dialogue. That might play a role, too. 🤷‍♂️