Literally taken, "one of the only" is an oxymoron. They're either "the only" (no "one of") venomous mammal because there are no others, or there are others in which case they're "one of the few" or "one of the many" depending how many others there are.

At any rate, there are a bunch of search results for "one of the only" saying it's a neologism with the same meaning as "one of the few". So we're probably stuck with it now, just like we're stuck with "begs the question" used to mean "raise the question" and "two times longer" used to mean "two times as long".