Pony

A historical American measure — one ounce, or roughly 30 ml.

In Jerry Thomas's 1862 Bar-Tender's Guide the pony (1 oz) and the jigger (1.5 oz) were the two standard portions. The pony has fallen out of common use but the vocabulary survives in the two-sided modern jigger, which is really a jigger-and-pony.