The mimic can use its action to polymorph into an object, creature, or back into its true, humanoid 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.

When polymorphing it can turn into any beast or humanoid of medium or small size.

The Elder Mimic adheres to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 10+STR). Ability checks made to escape this grapple have disadvantage.

Due to your evolution when you are in a form other than your true form you are indistinguishable from what ever you are polymorphed as giving you advantage on Deception checks when questioned what your true form is. 

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

You can change your form to be horrifying when you make an Intimidation check at advantage, all other charisma checks with anyone who sees this form will have disadvantage. 

Over time you have consumed enough magical storage devices from adventures you have learned to turn your stomach into one, allowing you to consume and store up to 250 lbs of stuff.

You can make a bite attack dealing 1d8 piercing and 1d8 Acid damage. 

Due to your time mimicking wood and metal you have learned to harden your flesh giving you natural protection, while unarmored your ac is 14 plus your Dexterity modifier. Because of your ability to change you appearance you cannot wear clothing or armor.

You have 60 ft. of Dark Vision.