Vanderbrute

Dopplers Miss. When a creature rolls a 1 on an attack roll against the vanderbrute, it will polymorph into the creature that just rolled. It must concentrate on this ability as if concentrating on a spell.

Keen Hearing and Smell. The vanderbrute has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Pack Tactics. The vanderbrute has advantage on an attack roll against a creature if at least one of the vanderbrute's allies is within 5 feet of the creature and the ally isn't incapacitated.

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., 1 target. Hit: 15 (2d10 + 4) piercing damage damage plus 4 (1d8) acid damage. If the target is a creature, the vanderbrute also regains hit points equal to half the damage dealt. 

Slowing Breath (Recharge 5-6). The vanderbrute exhales gas in a 40-foot cone. Each creature in that area must succeed on a DC 18 Constitution saving throw. On a failed save, the creature can't use reactions, its speed is halved, and it can't make more than one attack on its turn. In addition, the creature can use either an action or a bonus action on its turn, but not both. These effects last for 1 minute. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself with a successful save.