This question was in my head for a long time and the pokemon x and y trailers came out. Wasn't Giratina the first pokemon to mega evolve,due he can transform at certain point into his alternate form.He can only do it for a limited time I think.
Pokemon Question
And to answer your question, I don't think so since it wasn't called that in gen V and it also is always in one form depending on what dimension it's in. (from the movie at least :P)
It should be moved but to answer your question, no:
Giratina-O (Origin Form) required the use of a held item, much like Mega Evolutions. The difference is that unlike Mega Evolutions, changing-formes just rearranges base-stats. His incredible 120 Base Defense and Special Defense gets switched to his Attack and Special Attack, where as his 100 Base Attack and Special Attacks get changed around to 100.
So his Base Stats would be:
Normal Forme: HP150/ATK100/DEF120/SPATK100/SPDEF120/SPD90
Origin Forme: HP150/ATK120/DEF100/SPATK120/SPDEF100/SPD90
The same applies for Deoxys: Deoxys has multiple formes: Basic, Attack, Speed, and Defense.
All the formes do is change base-stats around. By that logic, Deoxys is the first Pokemon ever to have a mega-evolution.
Meag Evolution boosts their stats further and beyond. An Example is that Mewtwo has HP106/ATK110/DEF90/SPATK154/SPDEF90/SPD130.
With Mega Evolution Y, Mega Mewto Y's stats go to: HP106/ATK150/DEF70/SPATK194/SPDEF120/SPD140.
As you can see, Mewtwo's total base stat goes for 680 to 780: Instead of not losing anything and having his stats rearranged, Mewtwo magically gains an extra 100 base stats in his Mega Evolution.
Unless you mean lore-wise, in which case, Giratina changes forms based on its location, where it's Normal Forme outside of his dimension, and his Origin Forme within it. He is able to transform at will as long as he holds an item that is actually a piece from within his dimension. Whereas Mega Evolution is created from a held-item and a bond with his/her trainer.
So no, Giratina isn't the first pokemon to be able to mega-evolve.
But I just want to point out one thing: what you are calling normal form is called ALTERED form, so as to work as the opposite of his Origin form.
everything else is right.
forms are not mega-evolution
Bye <:)
Wrong subforum.