As far as we know, Maul is Darth Maul's pre-Sith name.
He was born a Nightbrother and has brothers with similarly evil-sounding names: Savage Opress and Feral .
Maul's last name may be considered "Opress" since that's the last name of his brother Savage; on the other hand Maul's mother, Talzin , doesn't seem to have a last name.
Not all Sith change their names when they change their title.
There is at least one other example (albeit from Legends) of a Sith who used his birth name: Revan / Darth Revan .
Most Sith change their names in order to help hide their identity (which was especially important for, say, Palpatine/Sidious).
Maul didn't need to change his name because he didn't have a public identity that needed to be kept separate from his Sith identity (and he was conspicuous enough with his tattoos, which he received as a boy).
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It may very well simply be Maul.
In Star Wars: Rebels , Mauls rejects the Sith and says this: Formerly Darth, now just Maul.
Since he no longer considers himself a Sith, and dropped the "Darth" title, it doesn't make sense that he'd keep his Sith name if it wasn't also his real name.
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We may never know.
He mentioned in Rebels that he had a name once, but he has long since forgotten it.
I suppose he had to leave it behind when Palpatine made him into Maul
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In the book Darth Plagueis, Maul is taken as an infant by Sidious from his mother.
The child was named Maul.
Palpatine, in his urge to know more about the dark side than what Plagueis teaches him, manages to make a visit to the world of Dathomir, where he encounters a Dathomirian woman who gives him a Zabrak infant simply known as Maul.
Palpatine later has Maul sent to Mustafar, where he could be nurtured at his young age in the dark side of the Force.
http://starwars.wikia.com/wiki/Darth_Plagueis_(novel)
Later, Maul's existence is made known to Plagueis, who permits (and encourages)
Sidious to retain Maul, not as an apprentice, but merely as an assassin (akin to the relationship between Dooku and Ventris during the Clone Wars).
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In the book, 'Darth Maul, Lockdown', in the prison he was known by the name 'Jangannath' this could be his first name.
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The Wookieepedia Canon gives information.
Here's the exact text: "Their son was born on the planet Iridonia[3] and was given the name Maul—the same Maul who would later be feared and dreaded as Darth Maul, Dark Lord of the Sith as the Sith apprentice to Darth Sidious." http://starwars.wikia.com/wiki/Maul's_father
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When a person becomes a Sith, he changes his name. For example:   Palpatine took (or was given) the name Sidious  Count Dooku took (or was given) the name Tyranus  Anakin Skywalker was given the name Darth Vader.   What was Darth Maul's name before he became a Sith?   Note: the idea for this question came from  http://www.moviemistakes.com/member542/answers/page49
What was Darth Maul's birth name?
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<star-wars><sith><names><darth-maul>
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It may not be known what Darth Maul's original name was, although it is quite likely that Darth Maul's birth name was simply Maul or Maul Opress. Another theory is that his name was Jangannath, as this was how he was known in the book "Darth Maul, Lockdown". There are grounds for believing his pre-Sith name was simply Maul, but he may also have had the surname Opress.
His pre-Sith name is Maul, as far as we know. He may also have had the last name Opress. We may never know the name - it may have been forgotten. He was called Jangannath in prison so this could have been his original name.