Fluttershy, AKA Saddle Rager. can turn into a hulking monster with tremendous strength when she loses her temper.

Origin:

The incredible Saddle Rager is the strongest there is! Spruce Spanner was just a mild-mannered scientist before she had a 'little' accident. Her high-tech invention to treat animal illness overloaded, and she saved all of her patients at the expense of being exposed to mysterious radiation! Now, whenever Spruce sees the defenseless being mistreated (even just a teensy little firefly), she can't control her temper. She transforms into the Saddle Rager and makes sure those big meanies pick on somepony their own size!