A red herring is a rhetorical device that diverts attention from a line of enquiry, topic, or suspect. In literature it is a plot device to distract the reader from an unfolding element of the plot.
The use of "red herring" to denote this rhetorical device is commonly, but incorrectly, thought to be derived from the attempt an escaping criminal might make to distract police bloodhounds from his scent by rubbing a "red" (smoked) herring across his trail. This misunderstanding apparently arose in the nineteenth century. In fact, though, the term was used as early as 1697 in descriptions of fox hunting: To exercise their horses between hunts, hunters would create a trail by dragging a dead cat or a red herring along the ground; the dogs would follow the scent; and the horses would follow the dogs.
- Oxford English Dictionary s.v. "red herring"