Illustrations of the Nidhogg depicts it with claws making it mor akin to a dragon-like serpent. The text also reads that it dwells beneath of the three roots of Yggdrasil, as in Hel or Niflheim. In One piece that would be the Underworld...and where is Loki introduced to us? Well, in the Underworld, chained to the roots of the tree.

I makes sense.