Rhinoceros?
The horn is curved the wrong way (Subtle hoax?), and seems to be way too far up the skull (long mutant secondary horn?), plus it seems to lack the upward swinging back of the skull. I never realized how much rhinos’ skulls look sorta like primitive ceratopsians. [The most famous ceratopsian is a triceratops, but there are earlier ones with only one horn and more modest frills in the back of their skulls. But actual ceratopsians grew out the frill before growing out their nasal horns, as far as I know.]
But I’m no expert, and I dont have nice 3-D images of them to put next to each other. Definitely seems more rhino-like than horse-like, though.