This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Geryon (Greek)
In Greek mythology, Geryon, son of Chrysaor and Callirrhoe, the grandson of Medusa and the nephew of Pegasus, was a fearsome giant who dwelt on the island Erytheia of the mythic Hesperides in the far west of the Mediterranean. Geryon was often described as a monster with either three bodies and three heads, or three heads and one body, or three bodies and one head.
According to Hesiod Geryon had one body and three heads, whereas the tradition followed by Aeschylus gave him three bodies. Apart from these bizarre features, his appearance was that of a warrior. He owned a two-headed hound named Orthrus, which was the brother of Cerberus, and a herd of magnificent red cattle that were guarded by Orthrus, and a herder Eurytion, son of Erytheia.