The King in the Mountain
Also known as Bergtruckung in Continental Europe, this is probably the most famous sleeping hero myth: a long-lost hero, be him a powerful king, a valiant warrior or a charismatic leader, who is not believed to be dead and buried but merely asleep and awaiting the right time to return among his people to save them from certain destruction or to lead them to greatness.
The best known is without a hint of doubt King Arthur, resting in Avalon until Britain will need him again.
But there are other similar figures in the British Isles: the first is Owain Glyndwr (anglicised as Owen Glendover), the leader of the last Welsh uprising against the kings of England. After the uprising was crushed he disappeared completely. Though he's commonly believed to have died around 1415 popular stories cropped up telling how he was merely asleep and awaiting the right moment to free wales from the English yoke. The second is Fionn Mac Uill (anglicised as Finn McCool) the legendary warrior of Irish mythology. There are two different legends regarding him: the first one says he's asleep in a cave near Dublin, awaiting the moment Eire will need him the most. The second says he'll be reborn when the right time will come (reincarnation being a very well known belief among Gaelic people).
On the Continent the best known stories regard Charlemagne and Frederick Barbarossa and were gathered by the Brothers Grimm.
Other less known myths have other great historic figures as main characters: Ogier the Dane, one of Charlemagne's best knights and owner of the legendary sword Cortana, Costantine XI Paleologos, the last Byzantine emperor, said to be saved by an angel, turned into a statue and been placed in a cave under Constantinople and Matthias Corvinus (also known as The Just) are three well known examples.
Holy Roman Emperors are well represented: besides the aforementioned Charlemagne and Frederick Barbarossa othesr are said to be sleeping in a hidden cave under a mounatin. They include Otto the Great, Henry the Fowler and Charles V of Hapsburg.
"Louhi spoke in riddled tones of three things to achieve: find and catch the Devil's Moose and bring it here to me. Seize the Stallion born of Fire, harness the Golden Horse. He captured and bound the Moose, he tamed the Golden Horse. Still there remained one final task: hunt for the Bird from the Stream of Death"
-Kalevala, Rune XIII-