A monster is a type of fictional creature found in horror, fantasy, science fiction, folklore, mythology and religion. Monsters are very often depicted as dangerous and aggressive, with a strange or grotesque appearance that causes terror and fear, often in humans. Monsters usually resemble bizarre, deformed, otherworldly and/or mutated animals or entirely unique creatures of varying sizes, but may also take a human form, such as mutants, ghosts, spirits, zombies, or cannibals, among other things. They may or may not have supernatural powers, but are usually capable of killing or causing some form of destruction, threatening the social or moral order of the human world in the process.

Grendel is a character in the Anglo-Saxon epic poem Beowulf. He is one of the poem's three antagonists, all aligned in opposition against the protagonist Beowulf. He is referred to as both an eoten and a þyrs, types of beings from wider Germanic mythology. He is also described as a descendant of the Biblical Cain and "a creature of darkness, exiled from happiness and accursed of God, the destroyer and devourer of our human kind." He is usually depicted as a monster or a giant, although his status as a monster, giant, or other form of supernatural being is not clearly described in the poem and thus remains the subject of scholarly debate. The character of Grendel and his role in the story of Beowulf have been subject to numerous reinterpretations and re-imaginings. Grendel is feared by all in Heorot but Beowulf, who kills both him and his mother.