Star Swirl the Bearded

Star Swirl the Bearded was a Unicorn who lived during the end of the pre-classical era, before the three Pony tribes were united, and through the early years of the Classical era. He briefly served as a mentor to Celestia and Luna in magic, teaching them to master the Sun and Moon control spells which the Unicorns had received from Sola and Lycan respectively.

History
Star Swirl was born late in the pre-classical era, and quickly established himself as a gifted Unicorn. He studied Magic his whole life, becoming one of the most powerful Unicorns and creating approximately 200 spells. Eventually because of his raw magical power, he was chosen as one of the few Unicorns to sustain the spells that would grant them control of the Sun and Moon. Even with the burden of the spell, he continued to practice his own magic and create new spells, such as the amniomorphic spell; he also became the mentor of Clover the Clever, who became one of the founders of Equestria in what is now known as Hearth's Warming Eve, and signaled the beginning of the Classical Era.

As Star Swirl began to grow older, his magical power began to diminish. Rather than simply pass the role onto future generations, however, he decided to ask help from the Alicorns who lived in the Sun and Moon, Sola and Lycan. The two gave the ponies their daughters, Celestia and Luna, to rule over them. As they grew, Star Swirl took them on as his new apprentices in Clover's place. He taught them both the Sun and Moon spells, though he expected that they would both cast one of the spells each once they became strong enough.

Star Swirl died the day after Celestia and Luna both cast the spells for the first time; they later discovered Star Swirl had been experimenting with time-related magic, and had created a way to artificially extend his lifetime by sending copies of himself from the past to the future, since he had supposedly died shortly after Celestia and Luna were born; what had been mistaken for senility in his later years was in fact his past selves not being aware of recent events when thrust into the future. In honour of him, since he had gone to great effort to ensure he could teach Celestia and Luna the spells so Clover would never have to succeed him, an entire wing of the Canterlot Archives was dedicated to him, and he became known historically as the most important conjurer of the pre-classical era for his role in the formation of Equestria.