Etymology
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foal (n.)
Old English fola "foal, colt," from Proto-Germanic *fulon (source also of Old Saxon folo, Middle Dutch volen, Dutch veulen, Old Norse foli, Old Frisian fola, Old High German folo, German Fohlen, Gothic fula), from PIE *pulo- "young of an animal," suffixed form of root *pau- (1) "few, little."
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foal (v.)
"give birth (to a foal)," late 14c., from foal (n.). Related: Foaled; foaling.
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pucelle (n.)

"maid, virgin, young woman," mid-15c., especially in historical reference to Joan of Arc, the "Maid of Orleans" (called in Old French la pucelle from c. 1423), according to French sources from Vulgar Latin *pulicella "maid" (source also of Italian pulcella), diminutive of Latin pulla, fem. of pullus "young animal," especially a chicken (see foal (n.)), but there are difficulties with this derivation. Also, in 16c.-17c. English, "a drab, a slut; a wanton girl, a harlot."

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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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*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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filly (n.)
c. 1400, filli, fyly, "a young mare, female colt or foal," possibly from Old Norse fylja, fem. of foli "foal," from Proto-Germanic *fuljo, from PIE *pulo- "young of an animal," suffixed form of root *pau- (1) "few, little." Slang sense of "lively young girl" is from 1610s.
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horse (n.)

"solidungulate perissodactyl mammal of the family Equidæ and genus Equus" [Century Dictionary], Old English hors "horse," from Proto-Germanic *harss- (source also of Old Norse hross, Old Frisian, Old Saxon hors, Middle Dutch ors, Dutch ros, Old High German hros, German Roß "horse"), of unknown origin. By some, connected to PIE root *kers- "to run," source of Latin currere "to run." Boutkan prefers the theory that it is a loan-word from an Iranian language (Sarmatian) also borrowed into Uralic (compare Finnish varsa "foal"),

The usual Indo-European word is represented by Old English eoh, Greek hippos, Latin equus, from PIE root *ekwo-. Another Germanic "horse" word is Old English vicg, from Proto-Germanic *wegja- (source also of Old Frisian wegk-, Old Saxon wigg, Old Norse vigg), which is of uncertain origin. In many other languages, as in English, this root has been lost in favor of synonyms, probably via superstitious taboo on uttering the name of an animal so important in Indo-European religion. For the Romanic words (French cheval, Spanish caballo) see cavalier (n.); for Dutch paard, German Pferd, see palfrey; for Swedish häst, Danish hest see henchman. As plural Old English had collective singular horse as well as horses, in Middle English also sometimes horsen, but horses has been the usual plural since 17c.

Used at least since late 14c. of various devices or appliances which suggest a horse (as in sawhorse), typically in reference to being "that upon which something is mounted." For sense of "large, coarse," see horseradish. Slang use for "heroin" is attested by 1950. To ride a horse that was foaled of an acorn (1670s) was through early 19c. a way to say "be hanged from the gallows." Horse latitudes first attested 1777, the name of unknown origin, despite much speculation. Horse-pistol, "large one-handed pistol used by horseback riders," is by 1704. A dead horse as a figure for something that has ceased to be useful is from 1630s; to flog a dead horse "attempt to revive interest in a worn-out topic" is from 1864.

HORSEGODMOTHER, a large masculine wench; one whom it is difficult to rank among the purest and gentlest portion of the community. [John Trotter Brockett, "A Glossary of North Country Words," 1829]

The term itself is attested from 1560s. The horse's mouth as a source of reliable information is from 1921, perhaps originally of racetrack tips, from the fact that a horse's age can be determined accurately by looking at its teeth. To swap horses while crossing the river (a bad idea) is from the American Civil War and appears to have been originally one of Abe Lincoln's stories. Horse-and-buggy meaning "old-fashioned" is recorded from 1926 slang, originally in reference to a "young lady out of date, with long hair." To hold (one's) horses "restrain one's enthusiasm, be patient" is from 1842, American English; the notion is of keeping a tight grip on the reins.

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