I have just run across Michael Quinion’s post at World Wide Words about the phrase “red herring”; I have seen the implausible explanation that herrings were dragged across trails to confuse hounds (who would do that, and why?), and Quinion (with the help of Gerald Cohen, Robert Scott Ross, and the Oxford English Dictionary) clears the matter up. A seventeenth-century treatise by Gerland Langbaine on horsemanship “suggested a dead cat or fox should be dragged as a training-scent for the hounds, so that the horses could follow them”:
If you had no acceptably ripe dead animals handy, he added, you could as a last resort use a red herring…
Robert Scott Ross and the OED now trace the figurative sense to the radical journalist William Cobbett, whose Weekly Political Register thundered in the years 1803-35 against the English political system he denigrated as the Old Corruption. He wrote a story, presumably fictional, in the issue of 14 February 1807 about how as a boy he had used a red herring as a decoy to deflect hounds chasing after a hare. He used the story as a metaphor to decry the press, which had allowed itself to be misled by false information about a supposed defeat of Napoleon; this caused them to take their attention off important domestic matters: “It was a mere transitory effect of the political red-herring; for, on the Saturday, the scent became as cold as a stone.”
This story, and his 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.
So now you know. (There is considerably more at Quinion’s post, including a discussion of the phrase “neither fish, nor flesh, nor good red herring” and of what a red herring actually is.)