The Danube, as a deity existed earlier, in the Hellenic era too. Its name was Istros (Ιστριη), which meant strong and swift mediated from Thracian language. Istros was the river-god of the north and Scythia according to the Greek mithology. His parents were Oceanos and Tethys, his brothers were the Nile, Eridannus (Po) and Alpheus (Alfíos on the Peloponnesos)
Istros used to be the name of the lower section of the Danube, from the Iron Gates to the Delta. There was also a greek colony with the same name Istros south from the Delta, founded by Miletian merchants and settlers. Some say, thet Istros has a common root with Isar (German), Isére (French), Isarco (Italian) and the slav Bistrica. I’m not sure if it is true, but Greeks do not use this form for the Danube anymore.