The elder son of Cymbeline, a legendary king of Britain during the reign of Augustus Caesar. Both Guiderius and his brother Arviragus were stolen in infancy by Belarius, a banished nobleman, out of revenge, and were brought up by him in a cave. When grown to man's estate, the Romans invaded Britain, and the two young men so distinguished themselves that they were introduced to the king, and Belarius related their history. Geoffrey of Monmouth says that Guiderius succeeded his father, and was slain by Hamo.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894