I looked up Google images for mimic and noticed most take the form of chests, why not skip the middleman and take the form of why adventurers(prey) go for the chests? Why not take the form of currency directly? Do young mimics look like currency?
D&D had a coin 'mimic', call hoard scarabs. Beatles that look like gold coinage when motionless, and according to the book (
Draconomicon, 3rd edition) they live in dragon hordes and clean/eat off of the dragon's scales, keeping them clean, kinda like those fish that clean shark mouths I imagine. Apparently they bore into the flesh of adventurers if they try to pick them up, in addition to the usual bug tactics of forming large swarms.
2nd edition also had bigger mimics, "house hunter mimics" that can take the form of a whole house, with their offspring taking the form of sheds or presumably outhouses.
And although "a chest" is the go-to artwork for mimics, in D&D they can take the form of basically any object. Doors, levers, tables, whatever they are large enough in volume they can get away with, though I image they can't "shrink" very well either. I guess one could become a big pile of coins, but it'd be a solid pile that has coin shapes on the surface. Which in D&D terms is probably good enough, as mimics sweat powerful adhesive, so their prey only really needs to touch them at all to be effective.
The existence of mimics does raise a question though. Since a mimic probably cannot trick the things living in the dungeon, how many adventurers can a mimic really expect to run into? Spiders can run into flys often enough, but how does a mimic eat enough adventurers to sustain itself?