Not an easy question. For me, he is still human (Gerudos are still a human race). Even when the Triforce of power corrupts him, he is still a human. The beast form and his mighty powers only granted him because of the Triforce.
Without the Triforce of Power, he would just be the evil thief Ganondorf, a male Gerudo (maybe with a few special abilities).
Ganon is not a born demon. A true demon is born into his body. The Boar look alike of Ganon shows only his true character (like Link transformed into a Rabbit or a Wolf). So, no, in my opinion neither Ganondorf nor Ganon is a demon. He is a human granted with some mighty powers of the gods.
If Ganon is a demon, than, a well known deceased German dictator should go by that title too.