Welsh: Gwydr
According to Geoffrey of Monmouth, Guiderius succeeded his father, Cymbeline (Cunobel), as king of Britain in the first century AD. His brother was named Arviragus.
Britain had been paying tribute to Rome for several generations when Guiderius took the throne, and Guiderius decided to stop the practice. As a result, Rome invaded Britain. Guiderius died fighting the Romans, but his brother Arviragus continued the fight and was victorious.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138