Manimals are a glitch phenomenon and can be encountered in Red Dead Redemption.
Shortly after the release of Red Dead Redemption people have started reporting glitches where animals would spawn with the models of people. These human-animal hybrids include a Man-Bear-Pig, a Cougar-Man and, perhaps most famously, bird people. To this day, it is unknown what caused the glitch to occur. It should be noted that since the release of the game in 2010, the glitch has been patched in an unknown update (most likely the Title Update 1.02).