Board Thread:Theories and Speculation/@comment-25139119-20150807165411/@comment-26369387-20150810011318

Obito summoned Kurama in chapter 502. It was similiar to way Animal Path summoned rest of Six Paths of Pain during Pain's assault to Konoha.

Also, while they do exist physically, it doesn't change the fact that Tailed Beasts are made of chakra... Their cells, flesh and what not are, obviously, not same as any normal animal.