Liviathan

Sea Serpent

 Leviathan

Origin: Judaism

Leviathan is a creature with form of sea serpent in Judaism. It is referenced in several book of Hebrew Bible, including Psalms, book of job and the book of Amos. It is also mentioned in the book of Enoch.

In case of fire meat water, the Leviathan combine two of Earth's natural elements to take the live of innocent sailors(basically avatar working for the fire nation).

Sighting: Year 1200

Logic:

Avatar working for fire nation to conquer seas

(there are more type of leviathan this is just one in many. I will post about more of them soon)

(They are one in many content type creatures)