A small glass for liquor. In the U.S., a pony generally holds exactly 1 (U.S.) fluid ounce or about 29.6 milliliters. The word "pony" means a small horse, hence anything smaller than normal--in this case, a smaller shot glass
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}