In Ultima Online, the player was a container — one you couldn’t open, but which held your equipped items, your backpack which was the container you could actually see, etc. Because of the freeform “gump” ((According to UoGuide, “graphical user menu pop-up.” It was the term that was used at Origin back then, long-forgotten now expect maybe among the UO emu community. Basically, any UI window of arbitrary shape floating above the game. In UO, inventory systems did not use slots but free placement on a coordinate system.)) style containment system used in the Ultimas, you could position anything to any location in a container, which meant they were basically treated like maps, with coordinate systems in them.
Then we added mounts.
When you rode a horse, we simply put the horse inside the player, and spawned a pair of pants that looked like your horse, which you then equipped and wore.
When we first did this, however, we forgot to make the horse stop acting like a horse. Pretty soon there was a rash of server crashes because the horse inside the player was wandering around, picking up the stuff it found inside the player, rifling through the player’s backpack and eating things it thought were edible, and eventually, wandering “off the map” because the player’s internal coordinate system was pretty small, and the edges weren’t impassable.