Are nutria related to guinea pigs?

Yes, nutria (Myocastor coypus) are related to guinea pigs, as both belong to the order Rodentia. They share a common ancestor and are part of the same family, Echimyidae, although nutria belong to a different genus. Both animals are herbivorous and have similar body shapes, but nutria are significantly larger and are semi-aquatic, whereas guinea pigs are terrestrial pets.