Danube
"Danube" is an Old European river name derived from a Proto-Indo-European "*dānu". Other river names from the same root include the Dunaj, Dzvina/Daugava, Don, Donets, Dnieper, Dniestr, Dysna, Tana and Tuoni. In Rigvedic Sanskrit, "dānu" means "fluid, drop", in Avestan, the same word means "river". In the Rigveda, "Dānu" once appears as the mother of Vrtra, "a dragon blocking the course of the rivers." The Finnish word for Danube is Tonava, which is most likely derived from the word for the river in Swedish and German, "Donau". Its Sámi name "Deatnu" means "Great River". It is possible that "dānu" in Scythian as in Avestan was a generic word for "river": "Dnieper" and "Dniestr", from "Danapris" and "Danastius", are presumed to continue Scythian "*dānu apara" "far river" and "*dānu nazdya-" "near river", respectively.
Istria
Some scholars speculate that the names Histri and Istria are related to the Latin name Hister, or Danube (especially its lower course). Ancient folktales reported—inaccurately—that the Danube split in two or "bifurcated" and came to the sea near Trieste as well as at the Black Sea. The story of the "bifurcation of the Danube" is part of the Argonaut legend. There is also a suspected link (but no historical documentation in support of it) to the commune of Istria in Constanţa, Romania, which is named after the ancient city Histria, named after River Hister.