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...