If you transfer a girintina or battle a friend who has platinum with shayminboth in their different forms then they would look the same. Example: If you had platinum and facing somebody with diamond and you have shaymin sky form then the person playing diamond will only see the normal form however it still has the same stats. The same thing happens when trading pokemon. If you trade a girintina origin form to your pearl, then it will look like a normal girintina.
sources: I've done it before