Currently, his information is still pretty lacking. I'll get it all filled in sooner or later though.
Name: Prince Legis
Special tallent: Physics
Siblings/Relatives: Princess Celestia and Princess Luna (cousin)
Ruling the the life cycle of the stars and other celestial bodies as well as physics law, Prince Legis out-dates his Cousin Celestia by an unknown, but great amount of time. He is a very desolate individual, spending the majority of his life alone in the space of the universe, keeping law and order and thwarting chaos wherever it may rise.
Once every seven years on the summer solstice (summer sun celebration) he visits earth to ascertain that things are still functioning as they should be. Most only think of him as a myth, as he has never announced his presence to the world on his visits. He is viewed as nothing more than a bedtime story to little foals, often told that if they do not obey the laws of life, he may just come in the night and take their life away for the bad path they choose.
Of course, Pinkie Pie has always been living truth that this part of his story is not true.
At the very most, he may take you with him for a night and teach you a lesson about the order of things and why laws need to be followed. In the morning, you will wake and make the decision for yourself to believe whether or not everything you had experienced was real - or just a dream.
He does visit his cousins, Celestia and Luna, when he comes to Equestria, just as he visits all the members of his family. In the public eye, he will disguise himself as a normal, everyday earth pony, but in the grand halls of Canterlot Castle, he stands proudly in his true form.
Maybe, one day, you may catch a glimpse of him. The Majestic Prince Legis.