Where does this motif come from? It's everywhere. From Scandanavia to Japan everyone believed their storm god fought a giant snake creature.

Norse
>Thor vs Jormungander

Greek
>Zeus vs Typhon

Hittite
>Tarhunna vs Illuyanka

Canaanite
>Ba'al vs Litan

Israelite
>Yahweh vs Leviathan

Egyptian
>Set and Ra vs Apep

Babylonian
>Marduk vs Tiamat

Indian
>Indra vs Vritra

Japanese
>Susanoo vs Yamata no Orochi