Pity the porpoise: sometimes endangered, often thought to be a fish, usually confused with dolphins. From its very earliest appearances, the porpoise has been misconstrued: its name comes from the Latin words for "pig" and "fish."
The word porpoise first came into English in the 14th century through French, who had taken the name ultimately from the Latin words porcus ("pig") and pisces ("fish"). The porpoise likely got its name because its rounder body and snub nose reminded the Romans and then the medieval French of a pig's snout. From the beginning, the English word referred to any of the small toothed whales of the family Phocoenidae:
... we see many dyverse kyndez of Fisches, Delphines & porpasez layke tham.
— The Prose Life of Alexander, ca. 1440
Because they are similar in build to dolphins, they are often confused with them. According to the National Ocean Service at NOAA, you can tell them apart by looking at the nose, the dorsal fin, and the body. Dolphins typically have longer noses and bigger mouths, more curved dorsal fins, and sleeker, leaner bodies than porpoises do. The one thing they have in common is that neither the dolphin nor the porpoise is a fish: they are both mammals.