King Arthur

Arthur was King of England in the 8th century in an alternate timeline. He and Merlin battled Morgaine, but was eventually defeated and killed. Merlin placed Arthur's body and Excalibur in a spaceship and sent it to the bottom of Lake Vortigern in his native universe. Legends sprung up that Arthur had survived and would reawaken at the time of England's greatest need.

In the late 20th century, the Doctor and Ace discovered Arthur's spaceship and retrieved Excalibur. They later discovered that stories of Arthur's survival was just propaganda and that the King was really dead after all. (DW: Battlefield)

Other references
Arthur had a counterpart in the main timeline. (PDA: Wolfsbane)

Another version of Arthur existed in the distant future. (DWM: The Neutron Knights)

The people of Avalon believed that Merlin had brought Arthur to their world centuries ago. (MA: The Sorcerer's Apprentice)

On New Jupiter, the Medieval Zone of the EarthWorld tourist park was influenced by legends of King Arthur. (EDA: EarthWorld)