Board Thread:Theories and Speculation/@comment-2165692-20170426120148/@comment-734582-20170426141230

I think it's clear that Boruto summoned it.

The summoning animal thing was never really explained. It's hard to say. It's implied in the Anime that if you have no contract with an animal, you get summoned to your "spirit animal" (Ie with jiraiya). but in this case Boruto did sign some sort of contract (it's completely unclear how he even accomplished such a contract).

Though to me, it seems some sort of seal was broken when Boruto summoned the thing. Could it be that the Dark Chakra made some sort of dimension accessible that shouldnt be accessible?