A mimic is a fun monster, and essential in any good Dungeon.
As you look around the room, your eyes immediately fall on a large, ornate chest, sitting on a small plinth, a faint beam of sunlight illuminating the details in the carvings, the flecks of dust floating about serenely. You walk up to it, running your fingers over the intricate carvings, then just as you lean down to open the lid... it blinks at you. Roll Dexterity to jump out of the way as the Mimic lashes out at you!
However, any experienced player knows that a chest is, more often than not, not a chest. So, DMs learned to get sneakier; turning doors, wheelbarrows, or other simple items into Mimics (or, more accurately, Mimics into other simple objects).
However, what if a Mimic was pretending to be another creature? Say a level 5 party were exploring a dungeon, when suddenly... A Beholder attacks! Now that would certainly make any player evacuate (and in more ways than one). Obviously though, being a Mimic, it'd be a cakewalk for a party like that.
The rules do state that it can transform in to any object, and only while motionless.
The mimic can use its action to Polymorph into an object or back into its true, amorphous form. [...] While the mimic remains motionless, it is indistinguishable from an ordinary object.
So, can a Mimic achieve this, or is there another monster that serves this purpose?