Keen Smell. The bøyg has advantage on Wisdom (perception) checks that rely on smell.

Regeneration. The bøyg regains 5 hitpoints at the start of its turn.  If the creature takes acid, fire or radiant damage, this trait doesn't function at the start of the bøyg's next turn.

Frenzy. If the bøyg is reduced to half of its maximum hit points or fewer, it will enter a frenzy.  While in a frenzy, it will not use its venomous spit action, but all melee attacks will have damage increased by +2.

Shapechanger. The bøyg can use its action to polymorph into a Medium humanoid, or back into its true form.  Its statistics are the same in each form.  Any equipment it is wearing or carrying isn't transformed.  It doesn't change form it it dies.

Multiattack. The bøyg makes two attacks: one with its bite and one with its claws, or it makes one ranged attack with its venomous spit.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage and the target must make a DC 15 Constitution saving throw, taking 12 (2d8 + 2) poison damage on a failure, or half as much on a success.

Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Venomous Spit. Ranged Weapon Attack: +9 to hit, range 30 ft., one target. Hit: 7 (1d6 + 4) acid damage and the target must make a DC 15 Constitution saving throw, taking 12 (2d8 + 2) poison damage on a failure, or half as much on a success.