Other forms: anonyms
An anonym is a fake name someone uses in place of their real one. Samuel Clemens famously published books including The Adventures of Huckleberry Finn using the anonym Mark Twain.
You can use anonym to mean "pseudonym" or "alias," when you say things like "No one knows who Elena Ferrante really is — it's an anonym." The word can also describe a person who goes by a false name: "We have no idea who painted this mural, because it was done by an anonym over the weekend." This meaning, "nameless person," is the original one, derived from the Greek anonymos, "without a name."