Ganymede watering an eagle. From the Wikipedia: In Greek mythology, Ganymede (Greek: Γανυμήδης, Ganymēdēs) is a divine hero whose homeland was Troy. Homer describes Ganymede as the most beautiful of mortals. In the best-known myth, he is abducted by Zeus, in the form of an eagle, to serve as cup-bearer in Olympus.
This explains the eagle in the mosaic.