El Chucho or El ChuNcho?

That's "Chus". "Chucho" means "dog" in Spanish slang, there are worse nicknames for a bandit than that.

According to the IMDB, Volontè's character is "El chucho", and in my version of the film (DVD R2) the credit onscreen reads "El chucho, quien sabe?". However, most sources quote the film as "El Chuncho" instead. Go figure. Even "El chucho, quien sabe?" is problematic, as it is mis-spelled. Should be: "El chucho. ¿Quién sabe?".