A guinea pig is a roundish furry rodent often kept by Muggles as a pet.
- In Fourth Year, Professor McGonagall taught the Transfiguration Class how to do cross-species switches, changing Guinea fowl into Guinea pigs - although Neville's guinea pig kept its feathers (GF22).
Guinea: Possible confusion between African Guinea and the country Guyana in South America; also once used for any far-off place from England or Europe
Pig: refers to the fact that the creatures spend much time eating while making grunting noises like a pig
A "Guinea pig" also refers to any creature used as a test subject for experimentation
The idea that it would be easier to switch two creatures with similar-sounding names is also known as sympathetic magic.
In the Andes, Guinea pigs have always been raised for their meat. But when they were introduced into England in the 1500s, they were accepted immediately as exotic cuddly pets (source: Wikipedia).