Spider-like Hunter. The mimic starts combat with (3) Webbings.  The webbings stretch from one surface to another and occupy a 5*5 ft. space. To notice the webbings, a creature must exceed the total of the mimic's Stealth check with their Passive Perception or a Perception check.

Expansive (2 charges). At the start of its turn, the mimic regenerates all the destroyed Webbings and creates (2) Webbings in adjacent spaces. This trait does not apply to Webbings destroyed by magic damage and the spaces occupied by them. 

Transformative. At the start of the mimic's turn, before the Expansive trait triggers, all Webbings bulge, thicken and transform into Tentacles. Creatures grappled by the mimic become restrained. 

Webbing

(AC 10; HP 2; immunity to bludgeoning, piercing, poison, and psychic damage). When a creature starts or ends its turn in a Webbing's space, it is grappled. As an action, the grappled target can make a DC (10) Strength check, bursting the Webbing on a success. The Webbing can also be attacked and destroyed. 

Tentacle

(AC 15; HP 5 vulnerability to fire damage, resistance to bludgeoning damage). Each Tentacle occupies a space of 5*5 ft. square. The Tentacles adheres to anything that touches it. A Huge or smaller creature adhered to the Tentacles is also restrained by it (escape DC 13). Ability checks made to escape this restrain have disadvantage.  At the start of their turn, a creature restrained by the Tentacles is dealt 2 bludgeoning and 1 poison damage.

Tentacle. The mimic can choose between 2 actions with each of its Tentacles. To perform actions, Tentacles must be free and not grappling a creature.

• Strike. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 2 (1d4) piercing damage. 
• Grapple. reach 5 ft., 1 target. 

Jaws. The mimic can make one bite attack and a special devour action in a single turn

• Bite. Melee Weapon Attack: +4 to hit, 1 target . Hit: 3 (1d6) piercing damage. 
• Devour. The mimic makes one bite attack against an unconscious target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the mimic. During the next turn the Spider Mimic devours the creature, at the end of which it is is fully submerged into the mimic's body. At the end of the 2nd turn, the creature is fully consumed. 

Carry (requires a restrained creature). The mimic passes the restrained creature onto the adjacent tentacle closer to its jaws.  A successful DC 13 STR or DEX check, the creature escapes the restrain and falls prone on the ground. 

Attack of Opportunity. Makes a Strike attack against a creature leaving the tentacle's range