Starswirl the Bearded

Starswirl the Bearded is a Unicorn magician, who is mentioned by Twilight Sparkle as the "Father of the Amniomorphic spell". He never appears in the series, but he was only once mentioned in Luna Eclipsed.

Depiction in the series
No one knows how he looks like or how old he is, thought Twilight Sparkle said that she got his costume right. She explained to Spike that he's the one who created 200 different kinds of spells, stating he could be a powerful magician. Twilight Sparkle also said that he is the most important conjurer of the pre-classical era and has a shelf in the Canterlot library of magic named after him.

Trivia

 * Despite his name, Starswirl the Bearded isn't related to the Generation 3 and Generation 4 Earth Pony of the same name.