Like: Gallade, Magmortar, Electivire, Tangrowth, Togekiss, etc.
They were introduced in dp but most of them aren't part of the dp pokedex. As Omega Blade said, only new forms were introduced in platinum. All that is said is
>Giratina has received a new Forme known as the Origin Forme > Shaymin's new Forme is called Sky Forme and takes on a canine appearance > Rotom has new forms as well
Nothing is stated that new Pokemon have been introduced although some mons are uncatchable in platinum
Sources:
Experience https://pokemondb.net/pokedex/game/diamond-pearl https://bulbapedia.bulbagarden.net/wiki/Pokemon_Platinum_Version