Yup. You have werewolves, man.

They're cool and stuff

I have sims who are werewolves, but I've always had to use cheats; I don't think I've ever seen the leader of the pack.

That is the leader of the pack. They all look the same, and are always male.

The chance a wolf comes out depends on the amount of trees on the lot.

I one time made a sim who lived in a gigantic mansion on a hill (think Young Frankenstein's castle), and I surrounded the hill with a bunch of pine trees. Like an uncountable amount.

Wolves, both pack leader and pack mook, would be there pretty much 24/7. Bruno von Faust was a knowledge sim, so he didn't mind.