The girl is singing a very old griffin lullaby called "My Little Hero" to Spike (Yeah, My Little Hero: [something] is Magic. I realised that after I gave it that name, but I don't want to change it since it fits the image so well.) The lullaby would be in tone of Zenith by David Larsen ([link]
) which ispired whole story.
She sings this one to him, because he almost died in the mountains trying to save his friend, thus he is a little hero to her.
Some sketches and description of the story: [link]
Design of Spike ©
My Little Pony © HASBRO
Edit: Why are Brony musicians so amazingly talented? If do you want to hear something incredible, listen to this: [link]
And I recommend you to check ~WoodyJack
's whole channel. You won't regret it. At least I didn't.