In God of War Ragnarök, the giant serpent living in Midgard is Jörmungandr.
Jörmungandr, also called the World Serpent, is one of the most enormous creatures in the Norse realms. In the game, it occupies the waters around the Lake of Nine and can be seen coiled across the landscape. Its presence is tied to both Norse mythology and the game's larger story about fate.
The serpent is friendly toward Kratos and Atreus, although it communicates through a mysterious roar that only Mimir can partly interpret. Ragnarök expands the creature's role and connects it to the events surrounding Thor.
Fenrir is a giant wolf, Níðhögg is a dragon-like creature associated with the World Tree, and Garm is a dangerous hound. None of those names identifies the Midgard serpent.