There was no hard-coded engine Avatar; the sandbox had no such concept.

There were just things that you could possess, inputs to process and physics to affect.

This Avatar, its locomotion, logic state machine and animation was coded in-world, in C#.

You could be anything.

https://www.youtube.com/watch?v=0LvkaD3RsQQ

Avatar

YouTube
The cloning engine worked on everything. Might see if I have more videos around that show this at larger scale.