Stark Mountain (Japanese: ハードマウンテン Hard Mountain) is home to the legendary Pokémon Heatran. The Pokémon is only available on the second time through the mountain, after revisiting Buck's grandfather's house in the Survival Area. Players are required to have Pokémon with Strength and Rock Smash to proceed through the volcano. After encountering their rival, players will find Buck, who will travel with them through the mountain, up to the point where Heatran is encountered.
Basically complete Stark Mountain, then revisiting the place again after talking to Buck, Heatran will be there for you to catch.