Animal class refers to any wildlife, usually native to Sanctuary (i.e. not demonic or angelic). Some animals are even intelligent or semi-intelligent, and may even use magic. A corrupted wildlife form may or may not be an Animal, however, a savage demonic creature will not fall into this category.
Many animals are larger and stronger than Humans, and some of them (like Beasts) were even tamed by mankind. Others are hunted or just avoided contact with.
Khazra, which are corrupted humans in essence, count as Demons in the second game, but as Beasts in the third game.
Undead may be risen from Animal remains if needed.