A monster is anything- a creature, a being, a person, etc- that incites fear in the eye of the beholder. The term was used to describe ancient creatures in Greek myths who had the parts of various animals and the overall appearance of something not found in the natural world. Over the years, monsters became more and more human looking and the term began to be used to describe people doing things that were considered “monstrous” therefore making the term synonymous with evil.
One of the most famous human monsters was Frankenstein’s monster, cited as part of the birth of the zombie franchise and a revolution for the idea of what a monster is. That monster (who never had a proper name but is often referred to by his creator’s name) was all human but so horrifying in appearance that people were too afraid of him for him to ever be able to interact normally with other people. Another thing that Frankenstein’s monster did for the genre was introduce the idea that monsters, how frightening their appearance, could have human emotions.
Nowadays, just about anything can be a monster. I credit horror movies and video games for pushing the envelope on fear-inducing creatures that challenge what we typically imagine monsters to be. Some of them are very much like their Greek predecessors in that they’re all scary appearance with little character. Others, the more human ones, are like Frankenstein’s monster in that they’re capable of human emotions and the struggle for the audience is therefore connecting with this creatures who may feel very familiar. (This is where vampires, werewolves and the like fit in). And, given the general nature of the definition, we now see serial killers and other people who are completely human but do evil things lumped into the category.
It’s easily arguable that Halloween wouldn’t be Halloween without monsters. They’re in our costumes, our entertainment, our decorations, our celebratory activities, our music and everything else about the holiday. And because of this, Halloween is the great melting pot of archaic traditions, ancient religious rituals, old world mysticism and superstition, celebratory activities, Greek myths, horror fiction, literature, movies, video games, television shows, food, culture and uncategorized stuff that it is. Thanks to monsters, Halloween is the insanely fun holiday that I love most.