Danube is an Old European

Etymology
Danube is an Old European river name derived from the Celtic ‘danu’ or ‘don'[8] (both Celtic gods), which itself derived from the Proto-Indo-European *dānu. Other European river names from the same root include the Dunaj, Dzvina/Daugava, Don, Donets, Dnieper, Dniestr, Dysna and Tana/Deatnu. In Rigvedic Sanskrit, dānu means “fluid, dewdrop” and dānuja means “born from dānu” or “born from dew-drops”. 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 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.[9]

In Latin, the Danube was variously known as Danubius, Danuvius, Ister[10] or Hister. The Latin name is masculine, as are all its Slavic names, except Slovenian (the name of the Rhine is also masculine in Latin, most of the Slavic languages, as well as in German). The German Donau (Early Modern German Donaw, Tonaw,[11] Middle High German Tuonowe)[12] is feminine, as it has been re-interpreted as containing the suffix -ouwe “wetland”.

Romanian differs from other surrounding languages in designating the river with a feminine term, Dunărea.[3] This form was not inherited from Latin, although Romanian is a Romance language.[13] To explain the loss of the Latin name, scholars who suppose that Romanian developed near the large river propose[13] that the Romanian name descends from a hypothetical Thracian *Donaris. The Proto-Indoeuropean root of this presumed name is related to the Iranic word “don-“/”dan-“, while the supposed suffix -aris is encountered in the ancient name of the Ialomița River, Naparis, and in the unidentified Miliare river mentioned by Jordanes in his Getica.[3] Gábor Vékony says that this hypothesis is not plausible, because the Greeks borrowed the Istros form from the native Thracians.[13] He proposes that the Romanian name is loanword from a Turkic language (Cuman or Pecheneg).[13]