long hair style, originally of girls, 1950, from pony (n.) + tail (n.).
Entries linking to ponytail
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 The U.S. Pony Express began 1860 (and operated about 18 months before being superseded by the transcontinental telegraph).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.
tail n.1
"hindmost part of an animal," Old English tægl, tægel "a tail," from Proto-Germanic *tagla- (source also of Old High German zagal, German Zagel "tail," dialectal German Zagel "penis," Old Norse tagl "horse's tail," Gothic tagl "hair"), from PIE *doklos, from suffixed form of root *dek- (2) "something long and thin" (referring to such things as fringe, lock of hair, horsetail; source also of Old Irish dual "lock of hair," Sanskrit dasah "fringe, wick").
According to OED, the primary sense, at least in Germanic, seems to have been "hairy tail," or just "tuft of hair," but already in Old English the word was applied to the hairless "tails" of worms, bees, etc. But Buck writes that the common notion is of "long, slender shape." As an adjective from 1670s.
Meaning "reverse side of a coin" (opposite the side with the head) is from 1680s; that of "backside of a person, buttocks" is recorded from c. 1300; slang sense of "pudenda" is from mid-14c.; that of "woman as sex object" is from 1933, earlier "act of copulation" with a prostitute (1846). Of descending strokes of letters, from 1590s.
Tails "coat with tails" is from 1857. The tail-race (1776) is the part of a mill race below the wheel. To turn tail "take flight" (1580s) originally was a term in falconry. The image of the tail wagging the dog is attested from 1907. Another Old English word for "tail" was steort (see stark).