Croagunk venom is highly toxic to the vast majority of pokemon, a fact Croagunk take advantage of by jabbing other pokemon with their fingers and then running away until their foe faints – or worse, perishes. Wild Croagunk are known to use this poison to hunt pokemon far larger than themselves, eschewing small prey in favor of feasting on the occasional large beast to wander past their marshland homes. Despite this, their venom has no deleterious effect whatsoever on humans.
The origin of human immunity to Croagunk poison has been frequently debated by philosophers and occasionally biologists. It is indeed unique to humans; their closest living relatives, such as Primeape and Infernape, are easily felled by the poison of Croagunk. Humans are about the right size to be Croagunk prey, so some have considered it a defensive adaptation on the part of the Croagunk, for humans form large communities and take revenge whenever a pokemon slay one of their own. Others have supposed the opposite; that so many early humans fell prey to the Croagunk that only those immune to their poison were left alive.
It is common for Croagunk to poke, jab at, and manhandle their trainers, much to the trainers' chagrin, but they do this as a show of affection and treat other Croagunk the same way. Such prolonged roughhousing gives Croagunk trainers enough toxins in their bloodstream to kill a Mamoswine. But for them, and for other humans, it is only a painkiller, often one which eases pain from the very wounds these boisterous fighting pokemon cause. Croagunk poison may be harmless to humans, but their fingers still hurt them when jammed into one at a sufficiently high speed.