Since Greek sources preserved the name I would assume the -is/-ais appendage is them twisting the name to suit them, that leaves us with Tana or Tan, which is, well, just Danu or Dan. I didn't mention but Danu itself is possibly proto-Indo-European right now I don't wanna into my opinion on "proto-indo-european" language, and especially not into the so called Kurgan hypothesis...
. However the Dan/Tan, still could come from the Turkic teng
Let's jump one river to the east, the Volga's name preserved a supposedly proto-Slavic word, volga
(now in vlaga, vlaha, voloha forms in many Slavic languages), which means wetness.
The Volga's names in various Turkic languages is/was Itil, Etil, Atil, etc. also means river
or big river
- which very well might be the origin of the name of the Great Khagan/Shan-ju, Attila. Western (German) historians say ofc that he was named after the Gothic word atta/ata
= father. On that note noone considers the Hungarian atya = father (in Mordvin: ata...) because: "nooooooooo, Hungarians cannot be the descendants of the Huns, nooooo the Huns couldn't speak Hungarian because they spoke [insert whatever other language pulled out of their asses], saying Hungarians = Huns is a mortal siiiiiiiiin" - such cases
The previously mentioned Dentumoger is also called Etelköz
, a name coined in the 19th century on the basis of emperor Constantine VII's report. In his work De Administrando Imperio
, he reports about a region what he calls: Atel kai Ouzou
, Atelk Ouzou
, and Etel kai Kouzou
; and says the Hungarians (whom he calls on a couple of other names, like Hun, Turk, or Sabir) lived there. Etel
is simply the Volga, and -köz
= space between. But what Constantine meant under Ouzu or Kouzu? Who the fuck knows.