Reaper of Sheoldred 

Creature — Phyrexian Horror
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Whenever a source deals damage to this creature, that source's controller gets a poison counter.
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Cards that do something similar expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Pit Scorpion 91%
$0.09#28152
"Whenever this creature deals damage to a player, that player gets a poison counter."
Sabertooth Cobra 90%
$0.18#19465
"Whenever this creature deals damage to a player, that player gets a poison counter. The player gets another poison counter at the beginning of their next upkeep unless they pay {2} before that step."
Necrogen Rotpriest 86%
$0.12#6864
"Whenever a creature you control with toxic deals combat damage to a player, that player gets an additional poison counter."
Suq'Ata Assassin 83%
$0.57#21670
"Whenever this creature attacks and isn't blocked, defending player gets a poison counter."
+1 more matching line
Swamp Mosquito 83%
$0.72#18867
"Whenever this creature attacks and isn't blocked, defending player gets a poison counter."
Crypt Cobra 83%
$0.19#28153
"Whenever this creature attacks and isn't blocked, defending player gets a poison counter."