What is the beast that samneric see?

The identical twin brothers are called Sam and Eric, or Samneric for short, not Simon and Eric. In chapter 6: Beast From air Samneric simultaneously describe the beast, and I quote.... "It was furry. There was something moving behind its head--wings. the beast moved too---" "that was awful. It kind of sat up---" "There were eyes---" "Teeth---" "Claws--" "The beast followed us---" "I saw it slinking behind the trees---"