Artist’s description from DA:

*Fabula*, otherwise known as Mother Gaia or in other languages, roughly translate, All-Mother. The direct creation of Cael, she is responsible for everything on the MLP planet. She made all the other gods, all the flora and fauna, and all the sentient beings (more or less).

She has never shown herself to any mortal but there is a universal understanding of her presence, since all culture have record of her dating as far back as there are records. And all give pretty much the same depiction of her: a tall, thin mare who appears to constantly be carrying a foal. Other details vary from place to place.