In the universe of Star Wars, each of the Sith Lords have two names, one that they were born with and a title bestowed upon them after they join the ranks of the autocratic order of self-destruction. The notable exception to this, however, is Darth Maul, whose real name has never been revealed, and most likely never will.
Although Maul was largely wasted in The Phantom Menace and seemingly killed off unceremoniously at its climax, he was later revealed to have survived being bisected and falling down a vast shaft. His presence in the saga was then greatly expanded in complementary media, principally in animated TV series The Clone Wars and Rebels, and in the latter during an encounter with Ezra, he states he once had another name but no longer remembers it.
Maul’s backstory involves him displaying such fighting prowess from a young age that he was taken from his homeworld of Dathomir and raised to be a Sith, to the extent that he cannot recall his life prior to his apprenticeship under Palpatine. Thus, the identity of Darth Maul forms the entirety of who he is, with anything else he might have been called now cast aside as an irrelevance that no longer forms any aspect of his sense of self.
Given that his brothers had names like Savage and Feral, its fits that Maul might actually have been what he was called as a child, and Palpatine never bothered renaming him seeing as his moniker already aligned with the Sith theme of titles typically relating to an extreme negative of nature, emotion or action, such as Sidious, Bane, Tyranus, Plagueis and Tenebrous. However, whether or not this is the case is a moot point.
Unlike Darth Vader and Kylo Ren, whose redemptive arcs saw them casting aside the mantles of darkness signified by their assigned appellations and reclaiming their pre-corrupted identities, Maul had no previous self to salvage, and so the notion of who he was and what he was called prior to his becoming a Sith has no relevance, making him unique in Star Wars and a further facet of the mystery that made him such a favorite among fans.