"motherless calf in a herd," 1887, cowboy slang, of uncertain origin. It may have had an earlier, more specific meaning:

What is called a "dogie" is a scrub Texas yearling. Dogies are the tailings of a mixed herd of cattle which have failed of a ready sale while on the market. They are picked up finally by purchasers in search of cheap cattle; but investments in such stock are risky and have proven to be disastrous this winter. [The Breeder's Gazette, March 5, 1885]