Early on, creatures learned skills just like players did. In the alpha, this led to wolves who hunted bunnies, and some bunnies the bunnies escaped and learned a little, and eventually there were both dire wolves (their name actually changed to "wise elder wolf" or something) and vorpal bunnies. These latter ones killed many unsuspecting players in the alpha, and we had to remove the creature learning because it was bad UX> We left in a vorpal bunny template in the launch game, however, as an Easter Egg.
It was bad UX because people couldn't easily tell the difference between a vorpal bunny and a weak one. The name wasn't enough. UO had a "consider" style skill but no one used it (I forget the name, "evaluate" or something). Eventually, "consider" was streamlined in MMOs into color coding mobiles, a la WoW. But in UO, people relied on the creature's sprite to assess difficulty.
The energy vortex sprite was a different Easter Egg iirc. And sometimes, you could get energy vortices that were llama-shaped...