There once was a mimic that snuck into the shop of a gnomish haberdasher and was immediately attracted to the hats... the terrible... crazy hats. They were so garish and adorned the mimic could not resist but take their shapes. Time and time again, it would leave on the head of a customer, devour that head, and come back for another round. Eventually, the gnome caught on and started weaving the mimic's magic into their designs. Eventually, that magic blossomed further. The mimic could create its own hat and drive others to do its bidding of sorts. The gnome didn't survive.

Milliner Mimic

Tiny monstrosity (shapechanger), neutral

Armor Class 14(natural armor)

Hit Points 52 (8d4 + 32)

Speed 15 ft

Str 12 Dex 17 Con 18 Int 5 Wis 13 Cha 15

Skills Stealth +7

Damage Immunities acid

Condition Immunities prone

Sensesdarkvision 60 ft, passive Perception 11

Languages -

Challenge 3 (700 XP)

Shapechanger.

The mimic can use its action to polymorph into a magical hat or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only).

The mimic adheres to anything that touches it. A Medium or smaller creature adhered to the mimic is also grappled by it (escape DC 13). Ability checks made to escape this grapple have disadvantage. The mimic can activate or deactivate this ability as a bonus action.

False Appearance (Object Form Only).

While the mimic remains motionless, it is indistinguishable from a magical hat.

Grappler.

The mimic has advantage on attack rolls against any creature grappled by it.

Share Mimicry (Object Form Only).

A creature wearing the milliner mimic may cast disguise self and minor illusion (sounds only) at will.

Actions

Pseudopod.

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite.

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 4 (1d8) acid damage.

Mad Hatter (1/long rest).

The mimic targets one creature it can see within 90ft. The creature must succeed on a DC 12 Wisdom save or become charmed by the mimic for the next minute. While the target is charmed in this way, a hat similar to the last form the mimic took appears on its head, and a madness glows in its eyes. The mimic turns Invisible for the duration.

The charmed target must use its action before moving on each of its turns to make a melee attack against a creature other than itself that the mimic mentally chooses. The target can act normally on its turn if the mimic chooses no creature or if none are within its reach.

On each of the mimic's subsequent turns, it must use its reaction to maintain control over the target, or the spell ends. Also, the target can make a Wisdom saving throw at the end of each of its turns. On a success, the spell ends. If the mimic takes any Action, its invisibility ends, but the charm does not.