There is not a general formula for what constitutes a monster in Greek mythology. But there are basic guidelines for determining what is a monster and what is not. Monsters are usually vicious creatures that devour humans and wreak havoc with their lives. Sometimes they are used by the gods for their various purposes. Monsters can be used to discipline a society, keep them in check, to scare people, or to decimate or destroy an entire populated area. Monsters also may be used to keep people away from a specific area or object. Many times monsters can only be defeated by special heroes with unique gifts, or defeated only in a special fashion (such as burning by fire, or decapitation). Monsters usually are grotesque in appearance, with features that most humans would find fearful. Monsters are meant so scare people, so that those who defeat them are seen in a greater, more heroic light. Monsters also are usually lacking in intelligence. They aren't up to the human's level on that one, which explains how the humans are able to defeat them when overpowered on so many levels. The cunning that the monsters lack is their downfall. Monsters usually act on one level: kill. They don't seem to be able to put together complex thoughts. A monster on a par with human's intellectually would be a formidable foe indeed. But the monsters are stronger than humans and have many characteristics that let them dominate the average human, until a hero comes along and saves the day by conquering the monster. The most poweful and near invincible monsters cannot be defeated by humans alone, but require help from the immortal gods. Monsters have specific powers also, but these vary. They range from the Hydra's ability to regrow its heads, to the Gorgons turning those that gaze upon them to stone. Some monsters are fantastic in size, while others are murderous maniacs. Appearance usually indicates in what is and isn't a monster. If a figure is human-looking, no matter if he is insane and blood thirsty, most people would not consider him a monster, while a hideous beings even if he is good natured and kind, would be considered a monster by many. Monsters rarely act by themselves, but are used by a power greater than they are, usually fate or a god. Chances are, it's ugly, stupid, and it kills, it's a monster.