This creature is: mythological
Bucephalus was a fierce horse no one could tame... save for the young Alexander the Great. He realised the reason the stallion was so troublesome – the horse was distressed by its own shadow. He turned the horse so its head was towards the sun, and no longer being able to see its shadow, it calmed down enough for him to mount and ride.
‘Bucephalus’ means ‘ox-head’ in Greek, he got his name from the brand on his haunch which was shaped like an ox’s head.
Bucephalus proved himself to be an excellent horse and was ridden in many battles. When he died, Alexander founded and named a city, Bucephala, in honour of him.
Bucephalus was sometimes depicted with horns, presumably these were part of his armour or other such adornment, or possibly a confusion arising from his name.
As time passed and stories of Alexander the Great became increasingly mythologised (there’s griffons, mermaids and all sorts in some versions!), so too did Bucephalus. In Medieval manuscripts, the horse had become an altogether more fantastic – and carnivorous – beast…
Instead of having an ox head brand, he now was depicted with a three-horned ox-like head.
He was kept by Alexander’s father, King Philip of Macedonia, in his dungeons, and traitors would be sent into his cage to be devoured – Bucephalus liked eating people.
It was prophesied that whoever could tame and mount the ‘horse’ would become the future king. Alexander did just that, and rode Bucephalus in many battles, the ferocious horse-creature being able to eat some of their enemies.
Bucephalus wasn’t the only mythical creature sidekick Alexander the Great had – there was also a golden unicorn rabbit! You can read my post about that Here.
External links:
Bucephalus on Wikipedia
An article about Bucephalus on British Library’s Medieval Manuscripts blog, which has some pictures of the three-horned horse.
The only picture I came across of Bucephalus where he actually has an ox-head mark was this one on Wikimedia Commons