The Dragon (龍 Ryū), also called Dragon God (龍神 Ryūjin) is, according to Hieda no Akyuu's account in Perfect Memento in Strict Sense, the highest god of Gensokyo. According to that same account, the dragon looks like a serpent with hands and horns and that is thicker than a great tree that's thousands of years old, and is so long that it can block out the sky. Co-creator of Gensokyo alongside Yakumo Yukari and the other Youkai Sages.
Hakurei Shrine is generally attributed to the Dragon God by fans, since it is the nexus point of the Great Hakurei Barrier he helped create, but there is no concrete evidence in canon.
Might be based on the actual mythological dragon-god of the ocean and storms. Link: http://en.wikipedia.org/wiki/Ry%C5%ABjin