ISTROS (or Ister) was a River-God of Skythia and northern Europe (the Danube of modern Romania).
The most important neighbouring rivers were the Borysthenes (Dnieper) to the east, and Hebros in Thrake to the West.
Hesiod, Theogony 337 ff (trans. Evelyn-White) (Greek epic C8th or 7th B.C.) :
"Tethys bore to Okeanos the swirling Potamoi (Rivers) . . . Istros of the beautiful waters, Phasis [in a list of rivers]."
Aelian, On Animals 14. 23 (trans. Scholfield) (Greek natural history C2nd to 3rd A.D.) :
"The largest of the rivers of Europe, the Istros, rises from only a few springs and moves in a direction facing the first assaults of the sun. Later, many Rivers rise with one accord as though they were escorting him--for he is the King of the Rivers of that country--and flow perpetually, and those who live on their banks know each one by name. But as soon as they discharge into the Istros, the name which they had at their birth ceases to be used, they surrender it in his favour, all are called after him, and together pour their waters into the Euxeinos [i.e. the Black Sea]."
Philostratus the Elder, Imagines 1. 11 (trans. Fairbanks) (Greek rhetorician C3rd A.D.) :
"Swans scattered about, breathing sweet notes, will hymn the youth [i.e. Phaethon, who was cast from the chariot of the sun]; and flocks of swans rising aloft will sing the story to [the Rivers] Kaÿstros and Istros; nor will any place fail to hear the strange story."
- Hesiod, Theogony - Greek Epic C8th-7th B.C.
- Aelian, On Animals - Greek Natural History C2nd-3rd A.D.
- Philostratus the Elder, Imagines - Greek Rhetoric C3rd A.D.