Alien Species
The concepts of alien life and alien visitation of Earth are fundamental aspects of Intervention Theory. Several races of these beings are consistently described by credible abductees, depicted below by artist David Chace. Perhaps the best known are the iconic gray-skinned, black-eyed "grey" aliens, but also known are (from top left) Reptilians, Pleadians (or Lyrans), Anunnaki, (and from bottom left) Blue Men, Small Greys, Tall Greys, Grey-Human Hybrids, and Grey-Reptilian Hybrids. Not shown, but also well known, are a type that resembles bipedal praying mantises, and are appropriately enough  known as "Insectoids."

Intervention Theory posits that humans are a product of genetic engineering, combined with hybridization of the native primates of Earth with the Anunnaki, which strongly resemble humans in many ways. After all, the historical texts do say that the "gods" (aliens) made us "in their image, after their likeness."