a name for various types of small shark, mid-15c., dogge fysch, from dog (n.) + fish (n.). It is said to be so called because it hunts in packs. The wild dog was the image of sharks in classical antiquity as well, and Greek used kyon "dog" also for dogfish and sharks, especially the smaller kind.
But in the Mediterranean, among the Greeks and Romans of antiquity, closer contact with sharks had left an impression of vicious dogs of the sea. Thus, Pliny's canis marinus. The metaphor of the dog spread to the North to dominate the European image of the shark, from the Italian pescecane and French chien de mer to the German Meerhund and Hundfisch and English sea dog and dogfish. [Tom Jones, "The Xoc, the Sharke and the Sea Dogs," in "Fifth Palenque Round Table, 1983," edited by Virginia M. Field, 1985.]
Greek galeos "dogfish or shark" perhaps is based on galen "weasel, marten," which also was a fish name.