This here is MoiRaa. She is a Nuada, which is an ancient race of ponies that act as guardian angels or watchers for specific ponies they are linked to. MoiRaa is the Nuada of Spitfire. Nuada have two forms: a Heaven (left) and Earth form (right). These forms are based off of sunlight, so during the day they would be in their Earth form. At night, their Heaven form. You are able to see the stars behind them in their Heaven form, which makes them almost entirely invisible at night. Very rarely do Nuadas reveal themselves, but in life or death situations they do. There is a lot more information about Nuadas and I will leave a link in a comment below if you want to read more about them. MoiRaa marks the second official Nuada ever created. The creator of Nuadas, @BluDraconoid , pioneered them with his Nuada LiSaa who is linked to Fleur de Lis.
Fun fact: MoiRaa is an adaptation of Moira, meaning Fate in English. In Greek Mythology the Moirai are The Fates, but thats as far as I researched :p

Find more info about Nuadas here:

http://bludraconoid.deviantart.com/art/Li-Saa-Fluer-de-Li-s-Nuada-469896712

Nuadas are a race of pony that was created by BluDraconoid.