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!
Always fun.
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 mimic's Shapechanger trait does 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?
Best Answer
No,
Mimics can only strictly turn into objects, that means things like, doors, door frames, key holes, keys, key rings, door knobs, knockers, peep holes- and those are just the door-and-other-doory-objects Mimics.
But...
Since a dead creature is considered an object, they can turn into a dead creature, if you want. It should be noted that they don't get the creature's stats and attacks.
Creatures that shapeshift
There are many creatures that can assume the form of humanoids and beasts without casting spells: Metallic Dragons and Deva, come to mind. I won't list them here but you can look through the Monster Manual to find the ones with the "Shape Change" trait.
A creature that can cast Shapechange is also a good candidate for what you want, it is, however, a 9th-level spell.
However, it appears what you want to be able to do is to make the mimicking creature appear stronger than it actually is. Most creatures with Shape Change can only look weaker, one exception could be the Doppelganger- taking the form of, say, an Assassin.
For Beholders, specifically...
There is a fungus called Gas Spores found in page 137 of the Monster Manual. They can serve the purpose of mimicking Beholders, as they are described as:
Have fun freaking out your party!