Rat-tail \Rat"-tail`\, a. Like a rat's tale in form; as, a rat-tail file, which is round, slender, and tapering. See Illust. of {File}. Rat-tail \Rat"-tail`\, n. 1. (Far.) pl. An excrescence growing from the pastern to the middle of the shank of a horse. 2. (Zo["o]l.) (a) The California chim[ae]ra. See {Chim[ae]ra}. (b) Any fish of the genus {Macrurus}. See {Grenadier}, 2.