There are three others in Pandaria; Chi-ji the Red Crane to the south, Xuen the White Tiger to the North, and Niuzao the Black Ox to the west. It's not clear if these three follow the same cycle as Yu'lon, but all three share the same sparkling, ethereal appearance. We don't know where they came from. We don't know what they are. We don't know exactly how long they've been on Pandaria, although there is mention of Yu'lon in tales from the last Pandaren Emperor. We don't know what their ultimate purpose on Pandaria is, except to guard and watch over the world.
And one would think that a reputation grind dedicated to these ethereal creatures would answer at least a few of these questions.