Old Spider Legs is a mysterious creature sighted in America during the 19th century by a lumberjack.
According to the witness' report, as he was traveling through the forest he heard a quick and numerous galloping noise. It was then that he got a clear sight of a horse that had eight legs positioned like a spider’s legs. He got on his horse and chased after the beast, but he could not keep up and soon lost sight of it. Many have heard it since, but none have seen it again.
It might have possibly been a regular wild horse that had a twin that did not develop fully. Like the instance of a conjoined twin's parts growing on the surviving twin's body the animal must have received its extra appendages, which, in this case, were positioned like a spider's, and it then learned to use these to its advantage of running faster.
It could simply be a made up Lumberjack Tale.