Dark Summoner. It's not just an Xbox Live gamertag waiting to happen (xXxD4rkSummonerxXx, anyone?). It's also a pretty sweet social game for iOS. This undoubtedly Magic: The Gathering influenced RPG lets you summon creatures, complete quests, level up your skills...everything except battle monsters. Just kidding. Monster battles GALORE.
In fact, from a combat standpoint, Dark Summoner might be one of the closer games to Pokemon you'll find on the notoriously Pokemon-less App Store. But instead of Pikachu, you'll be summoning and training gargantuan iron clad beasts. "I choose you, Rancor!"