When the world was born, newly sculpted by Arceus' Thousand Arms, there was no metal. Then Groudon, Lord of the Earth and Day, cleaved the earth with a mighty claw, but the metal beneath was flimsy bronze. He dug down deep into the lava at the core, and there he lay three eggs
The first to hatch was the brother of fire, Heatran. He created the many common ores: iron, gold, silver and titanium. From the lava floes on Stark Mountain came all these rich and valuable metals for humans to use.
The second to hatch was Lanthan. A Pokémon of Earth, Lanthan took the rare and elusive ores without any names, and fixed them deep underground. It created the Lanthanite Mountain of East Tandor and went to sleep inside.
The last to hatch was Actan. A Pokémon of Shadow, Actan took the scarcest and most powerful ores that burn and hum with dangerous energy, and emerged from Actanite Mountain to teach the humans of their use. Thus do we have power not stolen from Pokémon or fueled by fire but drawn from the limitless source of metal itself.
And Actan saw that the humans had learned, and returned to its slumber.
The Expert of Myths and Legends in Venesi City tells a little more of their current state.
Ah! Actan and Lanthan! Twin brothers, the Rare Metal Pokémon. Each are supposedly a thousand years old. Legend has it they crawled out of the earth's crust and taught Man the use of metals. Supposedly another of their kind lives in Sinnoh and is known as Heatran.
Nowadays they are said to dwell within the volcanoes of Tandor. That's how Mt. Actanite got its name.
Note: Heatran is not present in-game, and is only included here for comparison purposes