Any thoughts on why Meowstic-M and Meowstic-F are considered the same Pokemon while Nidoran-M and Nidoran-F are not?
I think really it’s just a matter of the games’ history. In Red and Blue, gender was something that was unique to Nidoran, so having separate “species” was the only way they could do that. They could have retconned that, of course – reassigned the Pokédex numbers so that male and female Nidoran were formally the same species – but then there would technically be only 149 first-generation Pokémon, not 150, and something tells me that the idea of reducing the official number of Pokémon species would make Game Freak very uneasy. If you want an in-universe answer… well, it seems like they do consider them the same Pokémon. I mean, they’re both called “Nidoran.” Possibly the Pokédex separates them because they have different evolutionary paths, which Meowstic doesn’t.