Demons. They've made themselves to "appear" that way to sway the "scientific" community. You can't get "scientist's" to believe in angels/demons, but they will believe in aliens. Hundreds of years ago, (1400-1900) all the old wives tales and other supernatural occurrences said that there were these "things" with the heads of animals and misshapen bodies or animal heads on human bodies. At that point in time people feared wild animals because animals were still relatively dangerous. They didn't have the imagination we have today so the demons used what they naturally feared in their day to day lives. They made themselves look that way to strike fear in us. Fear is one of their main objectives. I also think that the ancient Egyptians worshipped demons and made drawings and statues of them, hence all the "gods" with anthropomorphic animals. Aliens are just the newest change in which they've undertaken to get us to believe in them and fear them.