What's the word for horse-headed human? (Opposite of a centaur)

Solution 1:

That's called Ipotane.

Ipotane — Google

Ipotane:

In Greek mythology, an ipotane was a member of a race of half-horse, half-humans. The ipotanes are considered the original version of the centaur.

— Wikipedia

Or reverse-centaur (1, 2) as suggested by Greybeard

Hayagriva: Hayagriva, also spelt Hayagreeva (IAST: hayagrīva, literally 'Horse-neck'), is a horse-headed avatar of the Lord Vishnu in Hinduism.

Hayagriva — Wikipedia

Solution 2:

According to Wiki, there are these specific named gods:

Indian: https://en.wikipedia.org/wiki/Hayagriva

Chinese: https://en.wikipedia.org/wiki/Ox-Head_and_Horse-Face

The closest I saw was supposedly a half-horse/half-human 'kinnara' from Indian mythology, but it had no sources and none of the art in the main article depicts anything like this. I am disinclined to believe it to be factual.

I am not certain if a creature beyond a specific god was ever made in any mythology. As such, if you need the word in a creative project, having your created word following the etymology of any other mythical creatures that feature in the project could be one way to go about naming it.