Black Houndling

Keen hearing and smell. The hound has advantage on wisdom (Perception) checks that rely on hearing or smell.

Pack Tactics. The hound has advantage on an attack roll against a creature if at least one of the hounds allies are within 5 ft of the creature and the ally isn't incapacitated.

Amorphous. The hound can move through a space as narrow as 1 inch wide without squeezing.

Spider climb. The hound can climb difficult surfaces. including upside down on ceilings, without needing to make an ability check.

Corrosive Form. A creature that touches the hound or hits it with a melee attack while within 5 feet of it takes 4 (1d8) acid damage. Any nonmagical weapon made of metal or wood that hit's the hound corrodes. After dealing damage, the weapon takes a permanent and cumulative - 1 penalty to damage rolls. If the penalty drops to - 5 the weapon is destroyed. Nonmagical ammunition made from metal or wood that hits the hound is destroyed after dealing damage. The hound can eat through 2 inch thick nonmagical wood or metal in 1 round.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft, one target, hit 7 (1d8 + 3) piercing damage plus 7 (2d6) acid damage. 

Acid Breath (Recharge 5-6).  The hound exhales acid in a 15 foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 21 (6d6) acid damage on a failed save, or half as much damage on a successful one

Split: When a Hound that is Medium or larger is subjected to lightning or slashing damage, it splits into two new hounds if it has at least 10 Hit points. Each new Hound has Hit points equal to half the original hounds, rounded down. New hounds are one size smaller than the original hound.