Etymology
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pony (n.)

1650s, powny, "a very small horse" (less than 13 hands in height), from Scottish, apparently from obsolete French poulenet "little foal" (mid-15c.), diminutive of Old French poulain "foal," from Late Latin pullanus "young of an animal," from Latin pullus "young of a horse, fowl, etc." (from PIE root *pau- (1) "few, little") [Skeat's suggestion, still accepted]. Compare, from the same source, foal, filly, Sanskrit potah "a young animal," Greek pōlos "foal," secondarily also of other young animals; Latin pullus "young animal," Lithuanian putytis "young animal, young bird."

A small horse, especially one of a small breed, as opposed to a colt or filly, words which indicate merely young horses. German, sensibly, indicates this animal by attaching a diminutive suffix to its word for "horse," which might yield Modern English *horslet. Modern French poney is a 19c. borrowing from English.

The Shetland breed of ponies are stoutly built, active and hardy, with very full mane and tail, and of gentle, docile disposition. In western parts of the United States all the small hardy horses (mustangs or broncos) used by the Indians are called ponies. [Century Dictionary, 1897] 

Meaning "crib of a text as a cheating aid," especially a translation of a Greek or Latin author used unfairly in the preparation of lessons (1827) and "small liquor glass" (1849) both are from notion of "smallness" (the former also "something one rides," a translation being something that enables a student to "get along fast").

As the name of a popular dance, it dates from 1963. The U.S. Pony Express began 1860 (and operated about 18 months before being superseded by the transcontinental telegraph). The figurative one-trick pony is 1897, American English, in reference to circus acts.

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pony (v.)

1824, in pony up "to pay," of uncertain origin; similar uses of pony or poney in the sense "money" date to late 18c. OED says from pony (n.), but not exactly how. "Dictionary of American Slang" says it is from slang use of Latin legem pone (q.v.) 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. Latin pone is the imperative of Latin ponere "to put, place" (see position). Related: Ponied; ponying.

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ponytail (n.)

long hair style, originally of girls, 1950, from pony (n.) + tail (n.).

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pullet (n.)

late 14c., polet, "young fowl" (late 13c. as a surname), from Anglo-French pullet, Old French poulette, poilette, diminutive of poule, poille "hen," from Vulgar Latin *pulla, fem. of Latin pullus "young animal," especially "young fowl" (source also of Spanish pollo "chicken," Italian pollo "fowl;" from PIE root *pau- (1) "few, little." Compare pony.

A cockerel is a male bird under one year old, a cock over one year old. A hen is a female over one year old and a pullet under one year old. [U.S. Department of Agriculture, "Practical Poultry Production," 1920]
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coach (n.)

1550s, "large kind of four-wheeled, covered carriage," from French coche (16c.), from German kotsche, from Hungarian kocsi (szekér) "(carriage) of Kocs," village where it was first made. In Hungary, the thing and the name for it date from 15c., and forms are found since 16c. in most European languages (Spanish and Portuguese coche, Italian cocchino, Dutch koets). Vehicles often were named for the place of their invention or first use (compare berlin, landau, surrey). Applied to railway passenger cars by 1866, American English. Sense of "economy or tourist class" is from 1949.

Meaning "instructor/trainer" is c. 1830 Oxford University slang for a private tutor who "carries" a student through an exam (compare pony in the student slang sense "translation"). Transferred sense in sports, "person employed to train athletes for a contest" is attested from 1861. A more classical word for an athletic trainer was agonistarch, from Greek agonistarkhes "one who trains (someone) to compete in the public games and contests."

All panelled carriages with seats for four persons inside, and an elevated coachman's seat, are designated coaches. The town coach proper, has windows in the doors, and one in each end, the quarters being panelled. [Henry William Herbert ("Frank Forester"), "Hints to Horse-Keepers," New York, 1859]
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*pau- (1)

Proto-Indo-European root meaning "few, little."

It forms all or part of: catchpoll; encyclopedia; filly; foal; few; hypnopedia; impoverish; orthopedic; Paedophryne; paraffin; parvi-; parvovirus; paucity; Paul; pauper; pedagogue; pederasty; pedo-; pedophilia; poco; poltroon; pony; pool (n.2) "game similar to billiards;" poor; poulterer; poultry; poverty; puericulture; puerile; puerility; puerperal; pullet; pullulate; Punch; Punchinello; pupa; pupil (n.1) "student;" pupil (n.2) "center of the eye;" puppet; pusillanimous; putti.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit potah "a young animal," putrah "son;" Avestan puthra- "son, child;" Greek pauros "few, little," pais (genitive paidos) "child," pōlos "foal;" Latin paucus "few, little," paullus "little," parvus "little, small," pauper "poor," puer "child, boy," pullus "young animal;" Oscan puklu "child;" Old English feawe "not many, a small number," fola "young horse;" Old Norse fylja "young female horse;" Old Church Slavonic puta "bird;" Lithuanian putytis "young animal, young bird;" Albanian pele "mare."

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Shetland 

group of islands north of Scotland, from Old Norse Hjaltland; in reference to a type of pony, 1801; as a breed of sheep, 1794. Related: Shetlander.

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Sheltie (n.)

also Shelty, type of small, sturdy horse, 1640s, "Shetland pony," an abbreviation of Sheltand, a metathesis of Shetland. Or the word may represent the Orkney pronunciation of Old Norse Hjalti "Shetlander."

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cayuse (n.)

"horse, Indian pony of the northern Rockies," 1841, American English, said to be a Chinook (native Pacific Northwest) word; also the name of an Indian group and language (1825); of unknown origin.

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bidet (n.)

1620s, "small horse," from French bidet (16c.), a word of unknown etymology. Originally in French "a small horse, a pony," thus "a vessel on a low narrow stand, which can be bestridden for bathing purposes," a sense attested in English from 1766.

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