Maybe they don't have a natural form, per se.
Either that or they look just like Bob, from Accounting.
Has TSR, WOTC, or anybody else ever produced a picture of what the Mimic looks like when it is not imitating a treasure chest (or whatever)? It's described as "amorphous", but that doesn't help picture it much. Pictures of it as a chest usually give it teeth and a tongue, presumably to show it in mid-transformation from a chest to its natural form. So I assume it's got a mouth in its natural state? What else - legs? Or is it more of a ooze like critter?![]()

It looks like your laptop![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.