This paper deals with the dreadful non-human characters in literary and cinematic works Frankenstein and Alien (1979) respectively. By observing the Frankenstein monster, the cyborg and the xenomorph in the two literary outputs from the perspective of post-humanism, the current study aims to rethink the relations between human beings and their non-human counterparts. How does humankind respond to non-human beings? What research has been done on the sapient creature, the man-made robot, and the alien species historically? What are the meanings of monstrosity and contagion? And how do the two concepts function in the novel and the film? All these questions are to be addressed in the paper. The article expects to take a post-humanistic view on the two notions and to provide references for the future post-human studies. It concludes that in the two works, the monstrous and infective nature of the inhuman others that prompts human beings not to embrace their non-human counterparts reflects the same feature of humans themselves and proves the vulnerability of human-centered humanity.