Corpse Fawn Runner

Agility. The Corpse Fawn doesn't provoke opportunity attacks when it moves out of an enemies reach.

Trample. The Corpse Fawn can pass through creatures one or more sizes smaller than it. Each creature who's space the Corpse Fawn enters during its turn is targeted by the following effect.
Strength Saving Throw: DC 14. Failure: 8 (1d8+4) Bludgeoning damage, and the target has the prone condition.
It can only target any given creature with this effect once per turn.

Short lives. From birth, the Corpse Fawn will instantly decompose in 2d8 days.

Not meant to live. The Corpse Fawn cannot eat, drink, feel pain, or rest.

Determination. The corpse fawn will not stop running until it has found a corpse far away from its birthplace, at which point it will infect it and instantly decompose.

Ram. melee attack roll: +2 to hit, reach 5ft. Hit: 9 (2d8) bludgeoning damage +4 (1d8) Bludgeoning if the deer moved at least 20ft before making the attack

Infect. The deer bites into a non-decomposed humanoid corpse within 5ft and plants a tiny deer fetus in the flesh. The fetus will start eating the flesh and rapidly growing. In 3d4 rounds, the Fetus will have grown into a full Corpse Fawn (see that stat-block). If there are less than 5 corpses in the vicinity when it takes this action, this will birth a Corpse Fawn Runner instead.