Rupert of the Rhine

Prince Rupert of the Rhine was a military commander, a nephew of King Charles I and cousin of King Charles II of England. He was fighting in English Civil War on the side of the Cavaliers.

On 26 July 1643, he met the Eleventh Doctor, Amy and Rory in Bristol. He used his poodle Boye to stun his enemies. (COMIC: Dog of War!)

Prince Rupert led an 18,000-man strong army that fought against Oliver Cromwell at Marston Moor in the English Civil War. He was soundly defeated. (PROSE: Nightshade)

In 1648, travelling under the alias Richard Godley, he managed to meet with the imprisoned Charles I. (PROSE: The Roundheads)

Behind the scenes
While Mark Gatiss did not use the title "of the Rhine" in The Roundheads, he stated that Charles I was Prince Rupert's uncle, thereby identifying him beyond any doubt.