TIGRIS was a River-God of Assyria (modern day Iraq), in west Asia.

Other important rivers of the east included the Euphrates of Assyria, and Orontes of Syria, as well as the Ganges and Hydaspes of India.

Pseudo-Hyginus, Preface (trans. Grant) (Roman mythographer C2nd A.D.) :
"From Oceanus and Tethys [were born] the Oceanides . . . Of the same descent Rivers: Strymon, Nile, Euphrates, Tanais, Indus, Cephisus, Ismenus, Axenus, Achelous, Simoeis, Inachus, Alpheus, Thermodon, Scamandrus, Tigris, Maeandrus, Orontes."

Sources:

• Hyginus, Fabulae - Latin Mythography C2nd A.D.