Guinea pigs are rodents of the family Caviidae, which also includes capybaras (the largest rodents on Earth). Despite their names, these critters are neither pigs nor naturally found in Papua New Guinea.
Nobody knows exactly where the term guinea pig came from. One possibility is that it originates from a mistranslation of French Guiana in South America, where they were first found. Another theory is that they were originally traded for the price of a guinea – an old British coin – when they were first brought to England. The ‘pig’ part is also a mystery, but could have something to do with their short, stocky bodies or the squeaks and squeals they make.