The anima recorder is a musical instrument, much like an accordion, that allows a human to communicate with and even control a monster. It boosts the monster's morale and energy, and was invented by the Tochikans.
Julio uses one to do circus tricks, and Nayuta explains that the Tochikans used them in the ancient wars to assist their monsters in battle. Some people decided to use them to take control of their monsters and force them into battle, though. These types of anima recorders became known as dark instruments, the most notable being Moo's Horn.
When the Pendant scientists were creating Moo, they knew that a monster so powerful would be a danger if it were given free will, so they created the Horn as a way to force him to work for their side in the wars. It wasn't long, however, before they realized that Moo's dark spirit was far too powerful to be controlled. Within two years of his creation, Moo broke free of the Horn's control, but not before learning how to exert his own control over the Anima of others. Because of the dark instruments, Master Moo now knew how to amplify the darkness in others, causing their hatred to consume them.
Once Suzaku defeated Moo and the wars subsided, the people of Pangaea destroyed or hid any remaining anima recorders. Moo's Horn was left under the watchful eye of King Mao Mao, until it is stolen during the events of Monster Rancher EVO.