"As with many legends, El Cucuy has many different shapes and forms. The monster originated in Portugal and Galacia, where it was known as a Coco and appeared as a ghost with a pumpkin head. In Brazilian folklore, this creature is known as Cuca and appeared like a female human-alligator hybrid.
There is also El Cuco or El Coco, a variation used widely in Spain and Latin American to discipline children who disobey their parents. Indeed, Spanish romantic painter Francisco Jose de Goya painted “Que Viene el Coco” (“Here Comes the Coco”) in 1799, which portrays the creature as a shadowed humanoid.
In Mexico and the Southwestern United States, this monster is often referred to as El Cucuy, although El Cuco and El Chamuko (“the devil”) are also used.
As far as physical appearance, El Cucuy is much like the bogeyman. El Cucuy is an amorphous being, one able to shapeshift into various common animals or become a horrible, hairy monstrosity or even an unexplained shadow on a wall. It also can become very thin or small so that it can hide under beds, in closets, or even behind curtains. Some legends say that he has glowing, fiery red eyes and is about the size of a fairy or imp.
According to the legend, El Cucuy is considered the opposite of a Guardian Angel. It is attracted to the defiance and disobedience of children. The naughtier the child behaves, the hungrier El Cucuy becomes, until at last it devours the child.The child is then goes to a place of no return.
The idea behind El Cucuy is to temper a child’s behavior, much like Santa Claus checking his list, although the former legend uses fear to achieve its goal. In Mexico, child abductors (roba-chicos) are a big problem, as these horrible human beings abduct children off the street and sell them to interested parties. This fear is compounded when a child misbehaves and wanders off, becoming a potential victim. El Cucuy is then used to persuade children not to wander off, as a monster is certainly scarier than a man, at least in a child’s mind.
El Cucuy has a lot in common with other similar legends, like that of the bogeyman and the Sack Man, both of which enjoy abducting or devouring misbehaving children.”