The concept of the boundary transgressing animals is somewhat ridiculous to explain what animals and things humans find bizarre. Venus fly traps are flowers that eat bugs like a lizard. Lobsters are large arthropods that walk along the ocean floor. Pangolins have armor like a dinosaur. Platypuses are semi-aquatic mammals with beaks that lay eggs like a duck. I agree that all of these animals make humans feel weird as they are not easily classified or categorized in the broad general categories.
Monsters occupy different categories. Classic monsters from movie and literature occupy the space between life and death, human and animal, man and machine. Zombies, vampires, cyborgs, and of course beings that are supernatural and human. Giants in the bible are the sons of fallen angels and human women.
Grendel from Beowulf is a great example. He is the son of a scaly, female lizard monster and Cain, who murdered his brother Abel and was exiled from civilization by God. Cain’s clan, at least in the imagined Denmark of Beowulf, is an entire race of humanoid monsters. Grendel has fur and his mother has scales, putting them apart from the relatively hairless apes that are people. Grendel’s mother lives beyond human civilization in an untouched swamp that sucks up and drowns all men who enter (except the superhuman Beowulf).
Grendel, who is more human lives in the border lands of human civilization. He is neither man or beast, wanders about the moors, and raids the walled-off, civilized world at night for food. He eats both livestock, like people, but also eats people, cannibalism which is taboo in most human cultures. Grendel is half-human thus this might make him a demi-cannibal? Does Grendel himself howl as he himself knows he is violating this taboo? This argument of Grendel being an existential outcast reminds of John Gardner’s Grendel novel. Grendel works as Grendel himself is a boundary transgressing animal. He is feared by men like men were apprehensive about eating lobsters, giant ocean floor bugs, until lobsters became a luxury good to conspicuously consume.