1659, powny, from Scottish, apparently from Fr. poulenet "little foal" (1444), dim. of O.Fr. poulain "foal," from L.L. pullanus "young of an animal," from L. pullus "young of a horse, fowl, etc." German, sensibly, indicates this animal by attaching a dim. suffix to its word for "horse," which might yield Mod.Eng. *horslet. Meaning "crib of a text as a cheating aid" (1827) and "small liquor glass" (1849) both are from notion of "smallness" (the former also "something one rides"). As the name of a popular dance, it dates from 1963. Pony Express began 1847. Ponytail, girls' hairstyle, first recorded 1952.
1824, in pony up "to pay," said to be from slang use of L. legem pone to mean "money" (first recorded 16c.), because this was the title of the Psalm for March 25, a Quarter Day and the first payday of the year (the Psalm's first line is Legem pone michi domine viam iustificacionum "Teach me, O Lord, the ways of thy statutes").