Frotho I

Frotho I, also known as Frode Haddingsson, is one of the legendary Danish kings in Saxo Grammaticus' Gesta Danorum, where he has a substantial biography. He is known for slaying an unnamed dragon and his military campaigns financed by the dragon's treasure.
Frotho succeeds his father Hadingus to the throne and replenishes the war-drained treasury by slaying a dragon and winning its treasure. According to the Gesta Danorum, Frotho is told of an island dragon guarding a trove of treasure and warned to cover himself with bull hide when battling the beast to block the poison and to aim for the dragon's belly. The dragon drinks some water and, on its way back, is slain by Frotho, the trove taken for himself.
He uses the money to finance expeditions into the Baltic, where he wins victories with clever stratagems. After some trouble at home he campaigns successfully in Britain and captures London. He finally dies in a war against the king of Sweden.