What unique fear do Martians or other aliens represent, as opposed to
zombies, vampires, and other types of monsters?

Fear of the unknown

Explanation:

With zombies, vampires, and other monsters, we have some idea of how they work and what makes them tick, with vampires and zombies we can infer that they are mostly humanoid and other monsters can give us clues to their environments. Alien creatures however are hard to understand if you have no understanding of the environement or their technologies and if they happen to be more advanced than humans we also have thier complex and unknown technologies to scare us

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d. cassius cannot believe that a weakling like caesar has become so great.