Marion Louise “Mary” Dahl, was a TV actress who was born with systemic hypoplasia, a rare condition which kept her from ever growing.
Theory:

That’s what I think Cozy Glow was, she had a similar condition, which enraged her but she decided to use this to her advantage to fool everypony. And she has been pulling schemes for years, and the School Incident was just the latest and greatest scheme (Fact she sent a copy of Twilight’s teachings to Flim and Flam) that’s why she was sent to Tartarus, she was not a filly being locked up, but a deranged mare who was discovered to be pulling evil schemes in secret for years.
Heck, I wouldn’t be surprised if that voice of hers was fake.