Everyone forgets Gen 4 was also the first time alternate forms became a new staple in the mainline game series from rotom's appliances to shaymin and Giratina's held items for their origin forms.
There were a few Pokemon with alterative forms prior to Gen IV, not many though.
-Unown: 26 forms in Gen II(One for each letter in the English alphabet) with two more added in Gen III(Question Mark and Exclamation Point). Outside of appearances there is no mechanical difference between any of them outside of HGSS, where the two introduced in Gen III have higher potential in their Pokeathlon stats. The forms are not in any way interchangible.
-Castform: Changes form and type certain weathers, such as Harsh Sunlight/Extremely Harsh Sunlight(Fire), Rain/Heavy Rain(Water), and Hail(Ice), but not in Sandstorm, Shadow Sky, Fog, or Strong Winds. None of it's non-normal forms are usable outside of battle, even when there is weather on the overworld in the games with following Pokemon.
-Deoxys: Prior to Gen IV, its form was based on what game it was currently being stored in. Normal Forme in Ruby, Sapphire, Colosseum, XD, and Pokemon Box, Attack Forme in Fire Red, Defense Forme in Leaf Green, and Speed Forme in Emerald. Different stat spreads and appearances between all four, on top of entirely different level-up moves. In Diamond and Pearl onwards, all four formes are obtainable in any games they appear in, being able to be changed using meteorites(sometimes multiple, sometimes singular) found in different parts of every region in which they are coded into the games.
All that platinum really introduced was mid-gen new forms, that, unlike Deoxys in Gen III, were interchangable within a single title with its older forms via the use of hold items(Giratina), key items(Shaymin), or interacting with overworld objects(Rotom), though the latter could also be applied to Deoxys in Diamond and Pearl. Speaking of Diamond and Pearl, there was also Burmy/Wormadam, Shellos/Gastrodon, and Cherrim prior to Platinum as well.
