Helm mimics pose as well made or even magical helmets to make themselves desirable to adventurers. They seek out caches of treasure or armories to hide among valuable objects or other armaments.  Once picked up by an adventurer, a helm mimic will travel with its new owner.  By night, while the owner sleeps, the mimic will rummage through their belongings in search of food.  If discovered (usually when a new owner attempts to identify it) a helm mimic will often try to negotiate; offering to serve the adventurer as its headwear in exchange for regular payments of food.

Rather that wait for their bite to expose an enemy to their acidic saliva, helm mimics expectorate with surprising accuracy.  They are capable of shapechanging their respiratory system, drastically altering it to enable them to breathe in any environment.  They can also alter their "eye holes" through a combination of shapechanging and bioluminescence to emulate the the glowing glare of some dread power.  Due to their nature as headwear, they shield their wearers from psychic damage and attempts at intrusion into their minds.  Finally, they are incredibly perceptive, and will share their insights with a creature they have bonded with.

Variant: Mimic Helm

Some hat mimics are willing to serve as headwear for other creatures. A mimic acting as headwear gains the following trait:

Mimic Helm. The mimic can be worn as headwear, and functions as a necklace of adaptation while being worn.  While acting as headwear for another creature the mimic takes no actions of its own.  An attuned wearer can sense what the mimic senses as long as it it wearing the mimic. The wearer can also command the mimic to cast a spell from the mimic's Innate Spellcasting trait, requiring the spell’s normal casting time and using the mimic's spellcasting DC.

While wearing the mimic, whenever the wearer makes a Wisdom check, it can replace the number it rolls with a 15.  The wearer can also use a bonus action to use the mimic's Headbutt as an Unarmed Strike against a creature it has grappled, using its own Strength modifier and proficiency bonus for the attack roll and bludgeoning damage.  Additionally, checks made to forcibly remove the mimic from its wearer against their will have disadvantage.

The mimic is immune to game effects that would damage or destroy items with acid such as a black pudding's Corrosive Form trait.  Non-acid game effects that would damage or destroy items deal 1d6 damage to the mimic instead of lower its damage rolls.  If an attack or game effect would reduce the mimic to 0 hit points while it is being worn and its wearer has at least 1 hit point, the mimic drops to 1 hit point instead.  While the mimic has 1 hit point or less the wearer loses all benefits provided by the mimic.  The mimic is considered a destroyed item until its hit points rise above 1.