Bonehound

Aggressive. As a bonus action, the bonehound can move up to its speed toward a hostile creature it can see.

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

Devil's Sight. Magical darkness doesn't impede the bonehound's darkvision.

Multiattack. The bonehound can make one bite attack and one barbed tail attack. It can replace one of these attacks with a breath weapon

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) piercing damage plus 10 (3d6) fire or poison damage (the bonehound chooses the damage type when it hits). 

Tail. Each creature within 10 feet of the bondhound must succeed on a DC 14 Dexterity saving throw. On a failed save they take 14 (2d12) piercing damage and are knocked prone. On a success they take half as much damage and are not knocked prone. 

Breath Weapons (Recharge 5–6). The bonehound uses one of the following breath weapons.

Fire Breath. The bonehound exhales fire in a 15-foot cone. Each creature in that area must make a DC 14 Dexterity saving throw, taking 17 (5d6) fire damage on a failed save, or half as much damage on a successful one.

Poison Breath. The bonehound exhales poisonous gas in a 15-foot cone. Each creature in that area must make a DC 14 Constitution saving throw or be poisoned for one minute. The creature can repeat the save at the end of their turns, ending the effect on a success.