Miniatures of mimics are rare, at least partially because mimics can be anything. But the form they're best known for -- a chest -- only recently became a staple of the miniature world. Wizard of the Coast never produced one, but Reaper did, known as the Mockingbeast:
The mockingbeast isn't just revealing itself, it's leaping to attack by extruding a foot beneath it. Little beady eyes glare mischievously from the top. Fangs bristle from the top and bottom of the open chest that forms its mouth, and a tongue swirls inside. This is an old sculpt too, a medium-sized figure harkening back in the days when there were two types of mimics.
That's right kids, there were once two types of mimics: smart ones who would negotiate and big dumb ones that would just try to eat you. They were differentiated chiefly by size, with the smaller ones being the smarter of the two. This also made the bigger mimics far less successful, as they had an overwhelming tendency to masquerade as chests no matter how large they were -- which is enough to make most adventurers poke the 10-foot wide chest with a stick.
You can purchase this miniature at Amazon.