That's a great question.
I -think- the following trigger Beast Slayer damage:
Apes, Bears, Batraxes, Dragons, Manticores, Minotaurs, Mirmeks, Rats, Rexadons, Shapeshifters, Snakes, Spiders, Ursans, Vorators, Wolves, Worms, Wyverns, Yetis
True beasts are pretty obvious. The challenge comes from what counts as "semi-animals."
The wiki has long been outdated on this topic and with the new content, it would be nice to get a solid, 100% clear answer.