Definition from Wiktionary, the free dictionary
longtail (plural longtails)
- A common designation for an animal that has an unusually long tail relative to similar species.
- In Bermuda, an alternative name for the white-tailed tropicbird.
- A longtail boat.
- (colloquial) In the Isle of Man, a rat.
The colloquial usage derives from a relatively modern superstition that it is considered bad luck to use the word "rat".