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.
Researchers has different opinions about the origin of the Danuvios name. Some of them derives it from the Thracian word donaris, others say that the Celtic or Iranian Dānuvius is its root. According to the latter idea, the verb dā is the root which means flowing, drifting. It is therefore likely, that this same verb appears in Don, Donets, Dnieper, Dniester river names. In fact, the Romans conquering Pannonia took the name from the native Dacian and Celtic tribes living along the Danube. Later this version became the name for the whole river section, worldwide.
When the Fürdő Island was excavated in 1873, from the middle of the Budapest, a second Roman altar came to light, dedicated also for the Danube. It is 3 feet (1 meter) tall and 2 feet (0,5 meter) wide, and not known whether it is from the Pest or Buda side. Inscription: Danuvio Sacrum (V)etulenus (A)proni(anus) (L)eg, legionus, i.e. A votive gift for Danuvios from Vetulenus Apronianus, legion commander.