Guinea pig

The animal commonly known as a Guinea pig is neither from Guinea, a region in West Africa, nor is it a pig. It is in fact a rodent from South America. The ‘pig’ part is easy to understand because the breed was called cavia porcellus, the latter being Latin for ‘little pig’. The species name has been around since the mid-17th century, but its current figurative meaning as a person used in an experimental or untried capacity is first attested from 1913, according to the OED. Long before this, from the mid-18th century, young, inexperienced midshipmen in the Royal Navy were commonly referred to as Guinea pigs. At the same time, Guinea pigs were widely being used in scientific experiments, which may have prompted this nautical usage. The use of Guinea pigs in scientific experiments during the 18th century is hard to explain because rats and mice would have been far cheaper, and for this reason are much preferred by scientists today.