Dinopithecus spawn in the redwoods of Lost Island. And there are lots of them, trust me.

Though, as of right now, they also spawn in the redwoods of Valguero, not as much.

I would love if their spawn in Valguero was confirmed to be official and not a bug or anything, since a new creature in an old map is very refreshing.

More Dinopithecus Encountering Tips