Board Thread:Theories and Speculation/@comment-32617252-20180330195347/@comment-33308438-20180405190552

Tailed Beasts were stated to be demons in part 1. Kakashi had Tobi's soul in his body which let him attain Rikudou's power which let him attain Perfect SusanoO.

So yeah I guess you could say that, Gaara called Ichibi a 'monk spirit' in part 1.