Rhesus monkeys, also known as Macaca mulatta, are a species of Old World monkeys primarily found in South Asia. These primates have brown-gray fur and distinctive red faces, making them easy to identify. However, there are several synonyms used to refer to these monkeys, which include Bengal monkeys, Indian monkeys, and bonnet macaques. The term rhesus comes from Greek mythology, where a king named Rhesus owned a horse with similar red eyes. In scientific research, rhesus monkeys are often used as an animal model due to their genetic similarity to humans. Despite their significant contribution to scientific research, this species is also threatened by habitat loss and hunting.