Monster is a term for any number of legendary creatures that frequently appear in mythology, legend, and horror fiction. They are also a mainstay of Dungeons & Dragons and similar role-playing games. "Monster" usually, but not always, implies that these creatures are larger than human size. It almost always implies that the creatures are powerful and hostile to the hero, and must be overcome to succeed in the quest.
Occasionally, there are monsters who act out of legitimate motives and their monsterous appearance leads to serious misunderstandings. One well known example is the Horta in the Star Trek episode, "The Devil in The Dark". It is an ugly looking, but fully sentient rock like monster that could spray a powerful acid and was killing miners in the mines of a planet. However, when Capt. Kirk and Mr. Spock investigated the situation, they discovered that the creature was really desperately attempting to defend her young which were being inadvertantly killed by the miners. They inform the miners and a mutually beneficial agreement with the creature was reached to peacefully resolve the situation.
Some well known monsters are: