This pin was offered recently either through Hake's Americana or eBay - I can't recall which one now. Regardless, this particular pin is a hard one to find.
The horse depicted in the illustration is none other than Mickey's trusty steed Tanglefoot, who curiously enough never made an on-screen appearance. Tanglefoot came to life at the hands of the great newspaper strip artist Floyd Gottfredson, making his first appearance in a Mickey Mouse daily comic titled His Horse Tanglefoot. The story began on June 12, 1933 and ran some 102 strips in length.
Mickey's horse was also showcased in several children's books including Whitman's 1934 six set Wee Little Books series, seen here in a previous post, Blue Ribbon's 1934 The Mickey Mouse Waddle Book, and David McKay's 1936 title Mickey Mouse and His Horse Tanglefoot.