From Halopedia, the Halo wiki
A qothal is a large sea creature found in the oceans of both Alluvion and Beta Gabriel, human-colonized planets, having been seeded on both planets as part of the Conservation Measure. During the peak of the ecumene, Forerunner research hunters took pride in successfully chasing and capturing qothal. These hunters, known as dark-divers, would bravely attempt to extract a viscid fluid from the creature's muscular glands, which was highly desired by the Lifeworkers. Although the qothal were unharmed by the process, the Forerunner divers undertaking the task faced significant danger.
Colloquially called the "nightmare eel" by some, qothal are particularly found in the darkness of Beta Gabriel's deep oceans and are rarely seen in shallow water past the first year of their life. Qothal on Beta Gabriel typically spend most of their life hunting for food; most notably, a three-meter species of crustacean that reside in the labyrinthine caverns that cover much of the ocean floors of the planet. 
A qothal swimming near Fathom Station.
List of appearances
- Halo 5: Guardians (First appearance)