Personally, I don't understand why there is so much surprise. If you really think about it, your avatar is something you will look at a lot. So you naturally choose something you like. And people have tendency to chose live objects rather than say, a gigantic 3D number, particle system, or white light. There is not really much choice; humans (humanoid characters, like elfs), animals (& imaginary creatures & hybrids, four-legged animals are typically weird to control in 3D environment) and maybe robots.

I feel seriously old since I've been using furry fox avatar in SecondLife back in the 2006, and even then, this was fairly common.