O nome da Mula é auto-escolhido e refere-se à sua esterilidade
"Secondly, we did not know of your physical shortcomings, particularly the one that seemed so important to you, that you adopted the name of the Mule. We didn't foresee that you were not merely a mutant, but a sterile mutant and the added psychic distortion due to your inferiority complex passed us by. We allowed only for a megalomania – not for an intensely psychopathic paranoia as well.
Second Foundation
e
“Exactly, First Speaker. I wish I had thought of that expression. What was the Mule? A mutant. But where did he come from? How did he come to be? No one really knows. Might there not be more?”
“Apparently not. The one thing that is best known about the Mule is that he was sterile. Hence his name. Or do you think that is a myth?”Foundation's Edge