Toxrill, the Corrosive

Toxrill's third ability triggers when a creature an opponent controls with a slime counter on it dies for any reason, not just due to its toughness being decreased by Toxrill's second ability.

Toxrill. the Corrosive's second and third abilities apply to all creatures you don't control with slime counters, even if those slime counters came from a source other than Toxrill's first ability.

Another card, Sludge Monster, causes non-Horror creatures with slime counters on them to lose all abilities and have base power and toughness 2/2. If you control both Toxrill and Sludge Monster, those non-Horror creatures are 2/2 creatures with no abilities that also get -1/-1 for each slime counter.