Naturalist Bite can be found in a cage in The Slave Pens. The cage is located just before Quagmirran. When you arrive in the room he will yell, "Hey! Over here! Yeah, over here... I'm in this cage!!!"
When you open the cage, a champion, enchantress, and soothsayer will appear. Naturalist Bite will help attack his captors if you bring them close enough.