My sister and I were diving deep into the lore and rules for mimics and the most terrifying thing they could be, is either an entire planet or a humanoid. They might even be able to be a planet-sized humanoid. Or even a human-sized planetoid 😱
Play Shattered Pixel Dungeon. You can get special mimics that can disguise as literally everything. A door, strength potion, the way to the next level, basically anything that looks useful. And of course they only appear as soon as you've forgotten to check for them.
I've often thought it'd be fun to let a party have a tame mimic as a pet. Not helpful in battle unfortunately, unless one of your opponents decides to take a looting break.
My party do have a pet mimic. He's called Philip and is mostly there for comic relief although he has been known to help with puzzles and things occasionally
One D&D podcast I listen to had one be a house in Sigil. After they had dealt with the plot to use it for magic or something some of the party ended up feeding it animals much to the chagrin of the rest of the party but did save some other creatures living in the area from getting eaten.
It's always more fun when mimics team up. A couple of goblins bother the party at night, just enough to give them chase, then when they return, hop into their sleeping bag and boom, mimic.