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That would make sense if some Pokémon like Pidgey were unable to learn it.
Personally, I think that only Pokémon usable in sky battles should be able to learn fly. Basically any Pokémon that can fly or levitate can participate in these, except for smaller ones such as Starly and Fletchling.
I think Wooper learning Ice Punch isn't so farfetched. He can use ice moves, so he could freeze the air around him to form the shape of a hand. This might sounds a little forced, but I think it works. Same with Gastly learning Fire Punch. He can use Will-o-Wisp, so I assume he could create...