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The whole "nidorina not breeding" thing was made to fix stuff in game coding. If someone modded the game to make Nidorina and Nidoqueen in the Monster and Field egg groups like Nidorino and Nidoking are, then Nidorina and Nidoqueen would only be able to breed Nidoran Female and not the male one. This is why Volbeat and Illumise don't have evos.
