Gramps says that he has personally seen a horse skeleton in a Buenos Aires Natural History museum, which was discovered by Charles Darwin in the area, and is all but indistinguishable from a modern horse.
After some research, I think Gramps may be confused. I see nothing in Darwins writings that says he found a full horse skeleton, but he did finds the teeth of Equus Curvidens. He did, however, find a full skeleton of a giant anteater, which was about the size of a small horse. This skeleton he did haul back to be preserved.