Mimic

Mimics are shapeshifting predators able to take on the form of inanimate objects to lure creatures to their doom. In dungeons, these cunning creatures most often take the form of doors and chests, having learned that such forms attract a steady stream of prey.
Imitative Predators. Mimics can alter their outward texture to resemble wood, stone, and other basic materials, and they have evolved to assume the appearance of objects that other creatures are likely to come into contact with. A mimic in its altered form is nearly unrecognisable until protentional prey blunderers into its reach, whereupon the monster sprouts pseudopods and attacks. When it changes shape, a mimic excretes an adhesive that helps seize its prey and weapons that touch it. The adhesive is absorbed when the mimic assumes its amorphous from and on parts the mimic uses to move itself.
Cunning Hunters. Mimics live and hunt alone, though they occasionally share their feeding grounds with other creatures. Although most mimics have only predatory intelligence, a rare few evolve greater cunning and the ability to carry on simple conversations in Common or Undercommon. Such mimics might allow safe passage through their domains or provide useful information in exchange for food.

Monster Manual pg. 220

Mimic CR: 2

Medium monstrosity (shapechanger), neutral

Armor Class: 12 (natural armour)

Hit Points: 58 (9d8 + 18) 9d8+18

Speed: 15 ft

STR

17 +3

DEX

12 +1

CON

15 +2

INT

5 -3

WIS

13 +1

CHA

8 -1

Skills: Stealth +5

Damage Immunities: acid

Condition Immunities: prone

Senses: darkvision 60ft., passive Perception 11

Challenge Rating: 2

Shapechanger. The mimic can use its action to polymorph into an object or back into its true, amorphous from. 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 Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 13). Ability checks made to escape the grapple have disadvantage.
False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary object.
Grappler. The mimic has advantage on attack rolls against any creature grappled by it.
Magic Resistance. The mimic has advantage on saving throws against spells and other magical effects.

Actions

Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) 1d10+5 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 creature. Hit: 7 (1d8 + 3) 1d10+5 piercing damage plus 4 (2d6) 2d6 acid damage.

Mimic Colonies

Mimics imitate terrain and dungeon dressing to hunt for food. Rare specimens develop a deeper understanding of the world and can communicate with other creatures. In extremely rare cases, groups of these creatures band together, creating colonies. These bonded mimics cooperate to create larger objects than any lone mimic could approximate. A mimic colony can work together to form buildings, bridges, crystal formations, cliff faces, statues, and nearly anything it desires. Entire villages appearing out of nowhere might be composed of mimics!
Mimic Communication. Members of the colony develop telepathy and the ability to speak. While within 10 miles of the colony, any mimic can communicate telepathically with other creatures within 120 feet of it and can speak Common and Undercommon fluently (or two other languages of the DM's choice). The colony's offspring gain these abilities innately and can use them even away from the colony, as shown in the Juvenile Mimic stat block.
Confronting a Colony. A mimic colony's primary goal is survival. If threatened by a force the mimics can't overcome, they are willing to bargain. Mimic colonies have learned that adventurers they can't defeat can be bought off with information about nearby creatures or locations, hidden treasure (which the colony obtained from prior "food"), or even one of their own young.
If the colony's survival is threatened and it thinks it has a chance of surviving a fight, it can leverage its combined might using special lair actions. On initiative count 20 (loosing all ties), the mimic colony takes a lair action, causing one of the following effects; it can't use the same effect two rounds in a row:

• The mimic colony chooses up to three creatures with 300 feet of it. Each target must succeed on a Dc 15 Strength Saving throw or have its speed reduced to 0 until initiative count 20 on the following round, as pieces of the environment grasp the target. If a target fails the save by 5 or more, it is restrained instead for that duration.
• The mimic colony uses the Help action, aiding a creature of its choice within 300 feet of it.
• The mimic colony chooses up to three creatures within 300 feet of it. Each target must succeed on a DC 15 Dexterity saving throw or take 13 (3d8) 3d8 acid damage, as orifices appear on surfaces in the environment and launch caustic spittle.
• The mimic colony chooses a cube of nonmagical, inanimate material in physical contact with in. The cube can be up to 15 feet on a side. The colony reshapes that material however it likes. The transformation lasts for 1 hour.

When determining the difficulty of an encounter with a hostile mimic colony, consider the colony to be one additional creature of challenge rating 2.

Tasha's Cauldron of Everything

Juvenile Mimic

Tiny monstrosity (shapechanger), unaligned

Armor Class: 11 (natural armour)

Hit Points: 7 (2d4 + 2) 2d4+2

Speed: 10 ft , climb: 10 ft

STR

1 -5

DEX

12 +1

CON

13 +1

INT

10 +0

WIS

13 +1

CHA

10 +0

Skills: Stealth +3

Damage Immunities: acid

Condition Immunities: prone

Senses: darkvision 60ft., Passive Perception 11

Languages: Common, Undercommon, telepathy 120 ft.

Challenge Rating: 0

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary object.
Spider Climb. The mimic can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Actions

Bite. Melee Weapon Attack: +3 to hit, reach 5ft., one target. Hit: 1 piercing damage plus 2 (1d4) 1d4 acid damage.
Shape-Shift. The mimic polymorphs into an object or back into it's 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.

Icewind Dale: Rime of the Frostmaiden

Spitting Mimic CR: 5

Large monstrosity (shapechanger), neutral

Armor Class: 14 (natural armour)

Hit Points: 85 (10d10 + 30) 10d10+30

Speed: 20 ft

STR

21 +5

DEX

12 +1

CON

17 +3

INT

9 -1

WIS

15 +2

CHA

10 +0

Skills: Stealth +7

Damage Immunities: acid

Condition Immunities: prone

Senses: darkvision 60ft., passive Perception 12

Challenge Rating: 5

Shapechanger. The mimic can use its action to polymorph into an object or back into its true, amorphous from. 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 Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 16). Ability checks made to escape the grapple have disadvantage.
False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary object.
Grappler. The mimic has advantage on attack rolls against any creature grappled by it.
Magic Resistance. The mimic has advantage on saving throws against spells and other magical effects.

Actions

Multiattack. The mimic attacks three times: twice with its pseudopods and once with its bite.
Pseudopods. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) 1d10+5 bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.
Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 10 (1d10 + 5) 1d10+5 piercing damage plus 7 (2d6) 2d6 acid damage.
Spit Acid (Recharge 5-6). The mimic spits acid at one creature it can see within 30 feet of it. The target must make a DC 14 Dexterity saving throw, taking 32 (9d6+1) 9d6+1 acid damage on a failed save, or half as much damage on a successful one.

The result of Netherese experiments on ordinary mimics - that spits acid.