If I had to assume, the Nidhoggr (if we assume thats what it is) is the Earth God and Hell Serpent together. They destroy the first world and thats exactly what Nidhoggr is known for.Maybe Hadjrudeen actually ate Hraesvelgr after failing to eat Ace's fruit?
Anyway, im doubting these are the names Oda gives them, since he already drew both Fenrir and Nidhoggr, 2 things Loki has been theorized as and I doubt thats "just" the name of the fruits.
Its like Sun God Nika, theres a God name for Luffy's fruit.
For all we know this Serpent/Dragon fruit could be called the "Earth God Elbaf" or Dragon Dragon Fruit Model: Elbaf similar to
Human Human Fruit Model: Nika or something. This would help tie things back to even Dorry and Broggy's dialogue about God Elbaf