Question

Well beside the tornado pokemon why are all the flying types have to be normal too ?

Comments

There is a Pokemon that is a Pure flying type Tornadus and Tornadus-T.

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He already mentioned that... :P

Answer 1

Pokemon wants to mess with our heads? lol.
Its a hard thing to say, but after diamond and pearl, it would be because flying type pokemon lose there flying type half when they use roost. and if they didn't have their flying type half.... they would be a nothing or something.

Comments

'???' type

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Does not exist..

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Then how come Curse used to be a ??? type? And Eggs were ??? types?  And clearly, because I have a source from Bulbapedia, it clearly does exist. Arceus even has a color for it. http://bulbapedia.bulbagarden.net/wiki/%3F%3F%3F_(type)

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If a purely flying type uses Roost, then that pokemon becomes a Normal type for that turn. It says to on the Bulbapedia site for roost

Answer 2

Well, it does make some sense since the ones that are normal and flying are usually related to non flying birds in reality. But its not entirely bad: it negates ghost completely. The only down side is that it makes fighter regularly effective.
one example of this is...
-doduo and dodrio, ostriches