Grown from Steve's detachable tail (which came off in Comic 144), this creature is not yet named.
Its first appearance as an entity on its own, rather than a tail, is in Comic 145. Apparently Steve finds the growing reptile tasty, as he attempts to eat it.
In Comic 146, Steve's Tail is more developed, but has gained the desire to be friends with Beaver's tail. How does he do this? By attempting to cut it off.