I guess it could be Tannin after all.
In Jewish folklore, there is a Tannin:
http://en.wikipedia.org/wiki/Tannin_(demon)
Interestingly, Tannin is sometimes associated with Tiamat, another dragon in DxD.
Apparently, in the Bible where the English word "dragon" was used, the original Hebrew used "tannin."
Tanninim, on the other hand, is a river in Israel.
*Edit:
Actually, scratch that... On further research, the Jewish section of English wikipedia article on "Dragon" gives:
Knowing no Hebrew, my guess is that "תנינים" has some strange pronunciation that can be rendered both as "Tannin" and "Taninim."In Jewish religious texts, the first mention of a dragon-like creature is in the Biblical works of Job (26:13), and Isaiah (27:1) where it is called Nachash Bare'ach, or a "Pole Serpent".[13] This is identified in the Midrash Rabba to Genesis 1:21 as Leviathan from the word Taninim (תנינים) "and God created the great sea-monsters."