Shapechanger. The Not-Deer can use its action to polymorph into a Deer-like-humanoid hybrid, standing on it's hind legs or into a Deer, on all fours. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. 

Keen Hearing and Smell. The Not-Deer has advantage on Wisdom (Perception) checks that rely on hearing or smell.

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

Innate Spellcasting (humanoid form only). The Not-Deer's innate spellcasting ability is Charisma (spell save DC 13). It can innately cast the following spells, requiring no material components.
At will: darkness, detect thoughts, disguise self (any humanoid form), major image, nondetection
3/day each: cause fear, mirror image, false life, sleep, suggestion
1/day: dominate person, mass suggestion, modify memory

Terrifying Gaze: When a creature that can see the Not-Deer's eyes starts its turn within 30 ft. of the Not-Deer, the Not-Deer can force it to make a DC 14 Wisdom saving throw if the Not-Deer isn't Incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly terrified and cannot move until the end of their next turn Otherwise, a creature that fails the save becomes afraid of the Not-Deer and must flee its sight. The afraid creature must repeat the saving throw at the end of its next turn, using its turn to drop its weapons and flee on a failure or Ending the Effect on a success.
Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the Not-Deer until the start of its next turn, when it can avert its eyes again. If the creature looks at the Not-Deer in the meantime, it must immediately make the save.

Multiattack . The Not-Deer makes two Claw attacks in it's humanoid form or in Deer form, it can make one Gore, and one Bite attack.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 8 (1d10 + 3) piercing damage. If the target is a humanoid, it must succeed on a DC 13 Wisdom saving throw or become afraid of the Not-Deer until the end of it's next turn.

Claw (Humanoid form only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) slashing damage.

Gore (Deer form only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) slashing damage.  If the target is a humanoid, it must succeed on a DC 15 Strength saving throw or become knocked prone.

Wolf in Sheep's clothing (1/Day). The Not-Deer magically calls 3d6 deer to its aid. The called creatures arrive in 1d4 rounds, acting as allies of the not-deer and obeying its mental commands. The beasts remain for 1 hour, until the not-deer dies, or until the not-deer dismisses them as a bonus action.