Bulezau (but cooler)

Rotting Presence. When any creature that isn’t a demon starts its turn within 5 feet of the bulezau, that creature must succeed on a DC 15 Constitution saving throw or succumb to disease and become poisoned. While poisoned in this way the creature must make a DC 13 Constitution saving throw at the end of its turn or take 3 (1d6) necrotic damage. Once a creature has succeeded twice the poison ends, but the creature instead makes a DC 13 Constitution saving throw or takes 3 (1d6) necrotic damage while within 5 feet of the bulezau.

Standing Leap. The bulezau’s long jump is up to 20 feet and its high jump is up to 10 feet, with or without a running start.

Hunter’s Leap. Immediately after using its leap to move into melee range of a creature the bulezau can roll a contested Dexterity check against one creature in range, adding double its proficiency bonus to the roll. If the bulezau is successful and the creature is its size or larger, the creature is mounted. While mounted the bulezau can use its reaction to give the creature it has attached a disadvantage on any attack rolls or ability check, it can only attack with its tail, and if it chooses to release the grapple by leaping it can do so without provoking opportunity attacks. A mounted creature cannot directly attack the bulezau or attempt to escape the hold but can use its action to give others an advantage on attack rolls against it.

Sure-Footed. The bulezau has advantage on Strength and Dexterity saving throws against effects that would knock it prone.

Cough up. The bulezau convulses and throws up what seems to be a beetle-like insect as well as bile and stomach acid. The insect has 1 hit point 15 feet of movement and takes its turn on initiative count 0. The bulezau can only have two insects active at a time. Both the pool of vomit and the insect have the bulezau's Rotting Presence trait. once the insect dies it disappears into a pool of vomit that dissipates on initiative count 0, after which the bulezau can create a new insect.

Horn crash. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 7 (1d10 + 2) bludgeoning damage. If the bulezau enters the melee range of its target on the same turn it makes this attack it can add 1d6 to its damage.

Festering Claw. Melee Weapon Attack: +4 to hit, reach 5 ft., 1 target. Hit: 11 (3d6 + 2) slashing damage. 

Barbed Tail. Melee Weapon Attack: +5 to hit, reach 10 ft., 1 target. Hit: 10 (2d8 + 2) piercing damage plus 5 (1d10) necrotic damage. If the target is a creature, it must immediately make the saving throw described in Rotting Presence in addition to the beginning of its turn (if poison is not already in effect).