Sludge Monster

Whenever this creature enters or attacks, put a slime counter on up to one other target creature.
Non-Horror creatures with slime counters on them lose all abilities and have base power and toughness 2/2.

Magic Online Promos

Commander Legends: Battle for Baldur's Gate

Whenever Sludge Monster enters the battlefield or attacks, put a slime counter on up to one other target creature.
Non-Horror creatures with slime counters on them lose all abilities and have base power and toughness 2/2.

Innistrad: Double Feature

Innistrad: Midnight Hunt

Innistrad: Midnight Hunt Promos

If a non-Horror creature with a slime counter on it somehow becomes a Horror, the effect making it 2/2 and making it lose its abilities ends. Its base power and toughness will revert to whatever it was before that effect started applying, although other effects that modify power and/or toughness, such as ones created by Auras, counters, and the like may still apply. It'll also have whatever abilities it had before it lost them.

Sludge Monster's last ability affects all non-Horror creatures with slime counters on them, not just creatures that had slime counters put on them with Sludge Monster's first ability.