>>44098305It appears on two routes in Pearl only (not even in Platinum), in an event only hidden grotto in White 2 only, the Mirage Forest in ORAS and nowhere else. I think you could get it in the Dream World too, but only in one area and with a low encounter rate.
I’m guessing they kinda just forgot about it during fifth and early sixth gens, and when they realised how rare it was they decided to keep it that way as a joke.
