Well, it definitely wasn't on mIRC.
What you are referring to sounds like a GMUD (Graphical MUD (Multi-User Dungeon)) (and not to be confused with the MUD client "GMUD" which is text-only). There are many of these, and none of them run on any sort of standard. They come in all forms, from 3DVR (virtual reality/vector rendering) to 2D sprite movement. These generally require specialized clients to access the proprietary server, as again, no standards have been formed.
There are many Graphical MUDs out there, so give Go[/co...d]e
a try. The most popular appears to be Furcadia, but that's a different story for a different day.