Minotaurs are an unnaturally formed species, created after the Shokan people placed a curse upon the Centaurians, which transformed many of them, like Motaro, into a bipedal race.
In Motaro's Mortal Kombat: Armageddon ending, after he defeated the elemental creature Blaze atop the Pyramid of Argus, the Centaurians were finally freed of this wretched curse, and were able to revert to their prior form before seeking retribution on their mortal enemy, the Shokans, for this treachery.
Origin
This bipedal species that Motaro and his people are cursed to be transformed into is based on the Minotaurs of legend.
Trivia
As described in Motaro's Armageddon Bio Kard, the Minotaurs were included because the developers were faced with the decision of not including him in the game, or removing his back legs due to the difficulty of compensating for his unique half-horse body shape. They decided to remove his back legs with the explanation that a curse was placed on his race.