Funnily enough, Assajj Ventress it not a Rattatatotitaktatatouille, and Darth Maul is not a Zabrak.
George Lucas loves him some Dathomirians. So both got retconned into Dathomirians.
Maul is a zabrak because he looks like a zabrak. Ventress is a rattataki because she looks like a rattataki...
My philosophy is if it looks like a duck, and walks like a duck, then its a duck.