A mythical ghost monster to eats or kidnaps disobedient kids. Most kids aren't even scared of how he looks it's about what he does that make children fear him. There's even a lullaby representing "El Cucuy."
El cucuy is a folklore story most mexicans used to tell their kids when they weren't doing what they were supposed to. They would usually say, "Te va llevar el cucuy."
The legend used to haunt me because I was a gullible little kid. I tell my little sister that he/she will snatch her.
Moral of this story is stop misbehaving and start listening to your elders.