"one of the only" implies more extreme scarcity than "one of the few", so unless there is some other grammatical consideration I'm missing, saying "one of the only mammals" seems to point more to there being only a handful of species (less than a dozen in total?), while "one of the few mammals" might imply that we're talking about there being few such individual creatures around (which while probably also true I don't think is meant as the focus).