The "Mohawk"-styled mane of the horse reminds the Rube of the illustrations in Alice in Wonderland -- the only other place he's seen a horse with a mane sticking up straight like that. (Well, maybe not the only other place, but the place he remembers best.)

See also: Tenniel illustration from Rube's Junior Library edition of Alice Through the Looking Glass, which fascinated him as it appeared one horse was riding on another.