The dragon or snake who retained the waters and was slain by Indra.

In the early Vedic religion, Vritra (Sanskrit: वृत्र (Devanāgarī) or (IAST)) "the enveloper", was an Asura and also a serpent or dragon, the personification of drought and enemy of Indra. Vritra was also known in the Vedas as Ahi ("snake"), and he is said to have had three heads. The myth involving Vritra evolved over time as Indra's prominence at the head of the Pantheon faded and the Brahmins sought to glorify Vishnu.

see Vrtra