Mules can be packed with heavy loads to carry. I think they have more or less the same muscle strength as horses.
In games, you pack your mules with all the things you can't carry yourself. In New Leaf, people pack all their extra items and QR codes onto mules so their main character has space for new things. :3
Who ever thought of that definition? I call them "alts" as in "alternate character" like in any MMORPG.
Mayor name: J087
Alt name: M087
Simple~
As for suggestions:
Call them "Sets" (or Seth) for complete serie/set storage, and "Cloth" (or Cleath) for clothing/gracie stuff.
"Bonnie" for skeleton storage. "Floris" for hybrids. "Toilet" where you dump your **** (you fill it in).
The term "mule" is used in MMORPGs as well. It really depends on who you play with and what the popular term is in the game you're playing. There are many different colloquial terms used to refer to alternate characters, including "mule," "toon," and "alt," as you have mentioned.