Tragically, Rotom is not a legendary as far as the games are concerned.
Well, considering that you can only get 1 in the game, I'd say it would be a legendary. Period.
So in RBY and GSC, was Lapras considered Legendary?
In RBY, was Jynx considered Legendary?
In RSE, was the Beldum line considered legendary?
In RSE, was Castform considered Legendary?
Afraid not. :)
Rotom can change forms, it belongs to last part of Sinnoh part of National Dex so I don't see any reason why would it wouldn't be considered as legendary
I don't think that the 60+ pokemon [excluding Giratina] that are between Manaphy and Giratina are Legendary as well. :p
And, when discussing the National Dex in DPPt, it's just last of the normal pokemon simply because it was discovered that way. XD;
It was a recently discovered Pokemon as made evident by the miniminiminisubplot in Platinum regarding Charon and Rotom DX
So, technically speaking, it
isn't a legendary Pokemon.
Ordinarily, I'd close this, but I'll leave it open to see if there are any rebuttals XD;
And, no, I don't think this belongs in Pokemon General. Sorry!