I love this. It really opens the mind to other evolutionary possibilities, and how different our barn animals of today could have turned out. This isn't impossible at all - I could see a creature like this evolving to take down something like a giraffe or an elephant in herds, while still remaining big and fast enough to evade predators and protect their kill and kin. Perhaps this animal would have been built more like a jumper, though, so as to leap up and tear out the throats of its tall prey. Definitely using a wolfxhorse or bigcatxhorse crossover in a story.