Depends on what is being incarnated, Lady. Non-trinitarians can believe in different kinds of incarnation, too (up to and including the incarnation of the only person of the one and only God Most High, as modalists do for example); whereas binitarian theists can believe in the same 2nd Person of the one and only God Most High incarnating that trinitarian theists do. (Stonehouse is the only strictly binitarian author I myself am aware of, but he was also a Christian universalist for whatever that’s worth, and wrote a LOT on Christian universalism, most of which has been lost now.) Bi-theists, by contrast, could believe in one of the two Gods Most High incarnating. Ditto for tri-theists. Polytheists could believe in one of the strongest high gods incarnating, though those entities would still be creatures dependent on something substantially different from themselves for existence. Pantheists can believe in special avatars of the God Who Is All, or could believe we are all incarnations of God. Which is all completely aside from mere manifestations of God Most High (or some lesser lord or god) acting in the natural system.
(An incarnation is a special kind of manifestation, but not all manifestations are incarnations. Docetists believe Christ was the manifestation of some deity, perhaps even of God Most High, but do not believe Christ incarnated: born of a woman as a human baby, grew up as a human with a human natural history, able to die the death of a human, etc. Conservative and Orthodox non-Christian Jews might or might not agree that God Most High visibly manifested in a lot of famous and obscure Biblical stories, but usually reject the whole idea of incarnation, and certainly reject that Jesus was Incarnated. Some Christian Jews also reject an incarnation of God Most High, trinitarian or otherwise, though they may accept such an incarnation, trinitarian or otherwise; others go for a manifestation instead, or for an incarnation of something less than GMH, or merely for adoption of a creature not previously pre-existant. But getting back to incarnation instead of merely manifestation… 
)
There are lots of theoretical options on incarnation, which a person could believe; to give another example, high Arians typically believe in the incarnation of the greatest creature (the highest super-angel in effect), though not the incarnation of God Most High. Neo-Arians don’t typically believe in an incarnation per se, and tend more toward adoptionism (a creature empowered and given authority by God Most High, perhaps even up to God Most High status – there are a lot of “unitarian” variations), but they could in theory believe that some pre-existent creature, though less than the greatest creature up to then (or later?) became incarnated as Jesus.
There are more variations than this, too. I’m not sure yet which variety Eric has switched to; he started out as an Armenian Orthodox (or so I recall him telling us, though perhaps I’m mis-remembering – my apologies Eric if so), and they’re trinitarian (or historically have been anyway, and I haven’t heard different lately) but along with other “Oriental Orthodox” groups (mostly Ethiopian and Coptic Orthodox now, although one of the Syrian Orthodox groups goes this way) they disagree with other ancient trinitarian groups about the extent to which Christ’s humanity operated with His full divinity: the Oriental Orthodox believe His humanity was completely subsumed by His deity, although they are not docetists – they do historically affirm Jesus’ full humanity, like the Central Orthodox (Roman Catholics/Eastern Orthodox) do, and like the Church of the East does (the Nestorians, Jacobites, and other Syrian Orthodox.) All three ancient trinitarian groups recently got together in past decades and apologized for slandering each other, and officially agreed that while they have some slight but important differences on how the two natures work together, they do all believe in trinitrairan theism and the full humanity and full divinity of Christ.
Anyway, to put it shortly, there’s a lot of scriptural data, and a lot of metaphysical concepts, so naturally there’s potentially and actually a lot of disagreement historically about how best to put everything together. It isn’t a broad topic for the faint of heart to dive into! 
