Armadillo
Because they are always genetically identical clones, the group of four young is a good subject for scientific, behavioural or medical tests. They provide the same biological and genetic makeup, and then get different treatment. This is the only regular case of 'polyembryony' in the class Mammalia, where one fertilised egg splits into four embryos. It only happens in the genus Dasypus, and not in all armadillos. Other animals which have this reproductive method are parasitoid wasps, certain flatworms and various aquatic invertebrates.