She is Mothra

She smiled, but Twilight was near ready to cast a concussive spell. It might have a horn and wings, but the hole filled hooves, exoskeleton, and draping mane clearly told her this was no alicorn. She was a changeling, and a queen at that; and presumably a queen Twilight knew well.

The changeling nodded slowly, not at all reacting to the young alicorn’s spiteful glare and her glowing horn as Twilight put herself between the intruder and the still sleeping Spike.

“Please, I must have a word with you Miss Sparkle.”

“Not today Chrysalis!”

Twilight barked out as she mentally clicked on the spell to fire a blast that would hopefully blow the intruder into the next century, only for two tiny voices to cut off her concentration mid-thought.

“Her name isn’t Chrysalis”