Acon

Shapechanger. The gnoll can use its action to polymorph into a hyena or back to its true form. Its statistics change to match the hyena's in this form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.

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

Gnoll Form: Rampage. When the gnoll reduces a creature to 0 hit points with a melee attack on its turn, the gnoll can take a bonus action to move up to half its speed and make a bite attack.

Improved Critical. The sidekick’s attack rolls score a critical hit on a roll of 19 or 20 on the d20.

Battle Readiness. The sidekick has advantage on initiative rolls.

Hyena Form: Bite. Melee Weapon Attack: +2 to hit, reach 5 ft., one target. Hit: 3 (1d6) piercing damage. 

Gnoll Form: Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 4 (1d4 + 2) piercing damage. 

Gnoll Form: Spear. Melee or Ranged Weapon Attack: +4 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 5 (1d6 + 2) piercing damage, or 6 (1d8 + 2) piercing damage if used with two hands to make a melee attack.

Gnoll Form: Longbow. Ranged Weapon Attack: +3 to hit, range 150/600 ft., one target. Hit: 5 (1d8 + 1) piercing damage.

Extra Attack. The sidekick can attack twice, instead of once, whenever it takes the Attack action on its turn. The number of attacks increases to three when the sidekick reaches 15th level. If the sidekick has the Multiattack action, it can use Extra Attack or Multiattack on a turn, not both. 

Second Wind. The sidekick can use a bonus action on its turn to regain hit points equal to 1d10 + its level in this class. Once it uses this feature, it must finish a short or long rest before it can use it again. The sidekick can use this feature twice between rests starting at 20th level.

Defender. The sidekick can use its reaction to impose disadvantage on the attack roll of a creature within 5 feet of it whose target isn’t the sidekick, provided the sidekick can see the attacker.