Tanuki, or Japanese racoon dog, the animal nook, timmy, and tommy are actually based on (it's only localized to racoon); is a famous magical trickster figure in Japanese folk lore.
They liked tricking humans into giving them money by trading them leaves magicly cloaked to look like other objects. Sound familiar?
They are not evil however, just mischivous, and in modern iderations of the mythos they generally bring good luck and fortune to small businesses. People even place their statues outside their stores to bring in customers.
Nook also started out as a small town businessman, down on his luck, and eventually with his quick wit grew his company into a housing empire where he builds homes, furnishes buildings, populates towns, and mentors others in good business practices ala Timmy and tommy, who were his pupils, and Lyle the otter later on.
Lyle being a former conman who likely got in over his head working with one kitsune...speaking of, one of the biggest bits of evidence of Tom nook being a yokai is his bitter rivalry with said fox, Crazy Redd.
Crazy Redd was a character in the series known for running a black market of fake and real art and would regularly con the player character out of their money selling them overpriced items and fake paintings.
In the back lore of wild world and happy home designer we learn that when Nook was young he went to the city with hopes and dreams to earn his fortune, but his dreams were crushed when all his money was stolen from him by a business partner he trusted, thus why he took Timmy and tommy and eventually Lyle under his wing and helped them learn better business practices to avoid, or at least get themselves out of, a similar fate. In happy home designer we learn that Nook once worked with a fox when he was young and he would never make that mistake again. Redd also makes multiple references to nook in that game as well, making fun of his home, ect. You can infer from this that Redd was the fox who cheated him and likely was the same one who broke his spirit in the big city. Tanukis and Kitsunes (fox yokai) have a similar bitter rivalry in mythos, to the death sometimes even.
Kitsunes also have the power of illussion but are much more malicious generally and regularly trick people into thinking sticks and leaves are valuable objects or food, much like Redd tricks people with his paintings.
Inshort Tom nook is a yokai cause Japanese myth inspiration in his character and species. However, this falls under and relies on the same assumption that Redd is also a yokai.
