A deliberate distraction.
In a literal sense, there is no such fish as a "red herring"; it refers to a particularly strong kipper, a fish (typically a herring) that has been strongly cured in brine and/or heavily smoked. This process makes the fish particularly pungent smelling and, with strong enough brine, turns its flesh reddish.[8] In its literal sense as a strongly cured kipper, the term can be dated to the mid-13th century, in the poem The Treatise by Walter of Bibbesworth: "He eteþ no ffyssh But heryng red."
Until very recently, the figurative sense of "red herring" was thought to originate from a supposed technique of training young scent hounds. There are variations of the story, but according to one version, the pungent red herring would be dragged along a trail until a puppy learned to follow the scent. Later, when the dog was being trained to follow the faint odour of a fox or a badger, the trainer would drag a red herring (whose strong scent confuses the animal) perpendicular to the animal's trail to confuse the dog. The dog eventually learned to follow the original scent rather than the stronger scent. An alternate etymology points to escaping convicts who used the pungent fish to throw off hounds in pursuit.
According to etymologist Michael Quinion, the idiom likely originates from an article published 14 February 1807 by radical journalist William Cobbett in his polemical Political Register. In a critique of the English press, which had mistakenly reported Napoleon's defeat, Cobbett recounted that he had once used a red herring to deflect hounds in pursuit of a hare, adding "It was a mere transitory effect of the political red-herring; for, on the Saturday, the scent became as cold as a stone." Quinion concludes: "This story, and [Cobbett's] extended repetition of it in 1833, was enough to get the figurative sense of red herring into the minds of his readers, unfortunately also with the false idea that it came from some real practice of huntsmen."
Although Cobbett most famously mentioned it, he was not the first to consider red herring for scenting hounds; an earlier reference occurs in the pamphlet "Nashe's Lenten Stuffe," published in 1599 by the Elizabethan writer Thomas Nashe, in which he says "Next, to draw on hounds to a scent, to a red herring skin there is nothing comparable." The Oxford English Dictionary makes no connection with Nashe's quote and the figurative meaning of red herring, only in the sense of a hunting practice.
The use of herring to throw off pursuing scent hounds was tested on Episode 148 of the series MythBusters. Although the hound used in the test stopped to eat the fish and lost the fugitive's scent temporarily, he eventually backtracked and located his target, resulting in the myth being classified as "Busted".