It's difficult to say for sure. Huan apparently wasn't part of a particular species of talking, super-intelligent dogs, instead being a singular creature. Yet, why a Maia would choose that specific form is a puzzle. Boredom?
Then again, maybe he was akin to the Ents in a remote way. We are told that the Ents came about when Yavanna made an appeal to Manwë for guardians of the things she held dear, and that Yavanna's desire would
Quote:
|
...summon spirits from afar, and they will go among the kelvar and the olvar, and some will dwell therein, and be held in reverence, and their just anger shall be feared.
|
Silmarillion Of Aulë and Yavanna
So the Ents were "spirits" that had come to inhabit bodies in Arda. They were not mortal, and Maia appears to fit the bill for their identities. The fact that Huan seemingly wasn't subject to mortal death either might bolster the idea that he was a similar being fundamentally.