That's the important question right there!
What would a 1212.5 lbs/550 kg metal monster eat?
After doing some research, I found out where you can find a Metagross, which is Giant Chasm's rustling grass in Crater Forest in Pokemon Black and White (original and 2). What other Pokemon can you find there?
Mamoswine, Audino, and Clefable can also be found in the rustling grass patch. All 3 of these Pokemon weigh considerably less than a Metagross, meaning that even Mamoswine (641.5 lbs/291.0 kg) would get pinned down by the weight of Metagross, which is almost twice its weigh!
In-Fucking-Credible.
What else though? Let's take a gander at Yanmega, shall we?
Its Pokemon Platinum Pokedex entry states: Its jaw power is incredible. It is adept at biting apart foes while flying by at high speed.
What this would imply is that it would essentially rip off chunks of its prey Pokemon while fighting them. Which might sound brutal-er than Metagross. Maybe even rip off the head of its prey and eat it, based on its jaw power.