The modern horse is much larger than its ancestors thanks to controlled breeding by humans who liked big horses.
Even today killing and butchering a zebra is not without effort, although if you use a firearm it cuts out a lot of the preliminary stuff.
in school we butchered an elk using stone tools, much tougher than using my Kershaws but knowing the connection of the muscles we could minimize the heavy cutting and we used pressure flaked chips much more than handaxes.