Hmmm the left photo is 'shopped on the left side, all the way to the leg with the prosthesis. Also, the sky is entirely fake. I doubt the veracity of what that photo (1 of 3) is attempting to portray, mainly that a pony wears a artificial limb. I don't doubt that such a thing is possible however.
Snopes and
LSU and
YouTube (2 min 15 sec) agree she is real.