The Hulk costume is a popular costume for Halloween that is mostly worn by boys. It features the Hulk character, a Marvel Comics superhero. A girl looking to dress up as the character can choose to go as the female alternative character, She-Hulk.
Someone looking for a more unique look to their Hulk costume can choose one of several different 'iterations' of the character. The easiest of these to replicate would be Grey Hulk, which is how the Hulk originally appeared when he first transformed, or the Red Hulk or "Rulk", a different character which otherwise looks very similar.