Terra

She's the sister of Princess Luna and Princess Celestia. (I'm trash I know)

Princess Terra is originator of Blueblood's lineage???????

Princess Terra roams the Earth/Terra (The planet could be named after her) sometimes just to check in. She's able to still control the planet while on it but it causes her some strain. Maybe Cybele is her heir and she can roam the planet searching for problems/checking in on behalf of her mother. 

Princess Terra has her own disguised form just like Celestia does.

Ohhhh shit! Then on one of Terra's check-ins she meets a very charming Stallion and they fall in love. She eventually has to leave and she has to leave Blueblood behind, because of weird legal custody and the stallion refuses to live in a weird other worldly plane. He loves being on the planet, he loves *her*. So she abandons both of them. She was torn between choosing her lover and her destiny. She chose her destiny.