"of two different colors, having spots or patches of white and black or another color," 1580s, formed from pie (n.2) "magpie" + bald in its older sense of "spotted, white;" in reference to the black-and-white plumage of the magpie. Hence, "of mixed character, heterogeneous, mongrel" (1580s). Properly only of black-and-white colorings (compare skewbald).
"a horse marked black and white, a painted pony," 1860, from American Spanish pinto (adj.) "piebald," literally "painted, spotted," from Spanish, from Vulgar Latin *pinctus, variant of Latin pictus "painted," past participle of pingere "to paint" (see paint (v.)). The pinto bean (1916), is so called for its markings.