in japanses mytholigy he is a demon. along with many others.

this is my understandin but it may be wrong

the reason why kyuubi is so strong is because he is the oldest of them all.

its either 100 years per tail or 900 years per tail.

he is the oldest and has the most time to gather streangth.

and really like you said he is the strongest because the text says so, so your just gonna have to go with that.