True venomous stings, with hypodermic injection, are found in various invertebrate phyla, and in fish and reptiles among vertebrates—but never in birds or mammals other than the platypus (unless you count the toxic saliva of solenodons and some shrews that makes their bites slightly venomous). Among mammals, the male platypus is in a class of its own, and it may be in a class of its own among venomous animals too. The fact that the sting is found only in males suggests, rather surprisingly, that it is aimed not at predators (as in bees) nor at prey (as in snakes) but at rivals. It is not
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