I know it's something from D/P/Pt, like it is with all the other Sinnoh legends hiding in Hoenn, but unlike them, it doesn't say the name of the theme on its location page, Scorched Slab, on Bulbapedia. So does anyone know?
While I could not find an official name in the OST, the unofficial name of the track is Battle! Heatran. (99% sure it's unofficial). If you are wondering what the name of the track is in DPPt, it is Battle! Legendary Pokémon. This is the default theme for Legendary Pokémon like Cresselia (but not the Lake or Creation Trio) in the Sinnoh games.