The genus and family name derive from the Greek word alopex, meaning fox. Indeed the long-tailed thresher shark, Alopias vulpinus, is named the fox shark by some authorities. In some villages in the local islands of its habitat, it is known as quigano(kee-gan-oh), which directly translates to "long and mischievous".
The thresher shark is characterized by its large upper caudal fin. This tail fin may often be 50 percent of the total length of the shark. It has a short snout and large eyes placed forward on the head. The second dorsal fin is much smaller than the first. The thresher is a strong swimmer and can leap clear of the water. The jaws are small with small, curved sharp teeth without basal cusps or serrations. Colour varies from brown to black with metallic hues from above and irregular white markings on the underside. In Canadian waters sizes have ranged from 3.3 to 5.5 meters (10.8 to 18 feet) long. The maximum size recorded for this species is 6.1 meters (20 feet), however they generally are between 2 to 5 meters (10 to 16.5 feet) in length.