The story
From dog + fish. Compare Greek σκυλόψαρο (skylópsaro, “dogfish”), Latin canicula (“dogfish”, literally “little dog”), Italian pescecane (“dogfish”), French chien de mer (literally “dog of the sea”), German Meerhund (literally “seadog”) and Hundfisch (“dogfish”), and English seadog. For non-Indo-European cognates, see Maltese kelb il-baħar (literally “dog of the sea”) and Turkish köpek balığı (“shark”), likely calques from Indo-European languages.
Etymology from Wiktionary, CC BY-SA 4.0.