Pearl: For your fire type, take a Ponyta. Your Ponyta loves the open fields of Solaceon; therefore, it must be involved in all of the battles on Route 210 (the route above Solaceon) once you catch it, in order for it to eventually evolve.
SoulSilver: Take a Goldeen for your Water-type. Your Goldeen loves the colour gold, and therefore loves money and gambling, so it must always hold the Amulet Coin once you get it. In order for it to evolve, you must earn a total 500 coins in Voltorb Flip.