It's possible. Like Pikachu, Plusle and Minun, and Dedenne...they all have pretty similar appearances, don't they?

[Bless you Goro for defusing the situation, because the Plusle definitely seems a bit more at ease now, if no less sad than he was when Chihiro first made his post.]

If nothing else, I'm sure that Plusle and Minun must be related somehow, if not part of the same species. They seem to just be variations of one another.