Tail
Old English tæg(e)l, from a Germanic base meaning ‘hair, hairy tail’; related to Middle Low German tagel ‘twisted whip, rope's end’. The early sense of the verb (early 16th century) was ‘fasten to the back of something’.
wiktionary
From Middle English tail, tayl, teil, from Old English tæġl(“tail”), from Proto-Germanic *taglaz, *taglą(“hair, fiber; hair of a tail”), from Proto-Indo-European *doḱ-(“hair of the tail”), from Proto-Indo-European *deḱ-(“to tear, fray, shred”). Cognate with Scots tail(“tail”), Dutch teil(“tail, haulm, blade”), Low German Tagel(“twisted scourge, whip of thongs and ropes; end of a rope”), German Zagel(“tail”), dialectal Danish tavl(“hair of the tail”), Swedish tagel(“hair of the tail, horsehair”), Norwegian tagl(“tail”), Icelandic tagl(“tail, horsetail, ponytail”), Gothic 𐍄𐌰𐌲𐌻( tagl, “hair”). In some senses, apparently by a generalization of the usual opposition between head and tail.
From Anglo-Norman, probably from a shortened form of entail.
etymonline
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).
tail (n.2)
"limitation of ownership," a legal term, early 14c. in Anglo-French; late 13c. in Anglo-Latin, in most cases a shortened form of entail.
tail (v.)
1520s, "attach to the tail," from tail (n.1). Meaning "move or extend in a way suggestive of a tail" is from 1781. Meaning "follow secretly" is U.S. colloquial, 1907, from earlier sense of "follow or drive cattle." Related: Tailed; tailing. Tail off "diminish" is attested from 1854.
