Examples:
Fluttershy changes size according to her emotions: Fluttersize (and that tag would be also valid for unrelated size change images like giantlestia, derpysaur or appletini).
Pinkie Pie blows up, desintegrates and raises from her ashes when the going gets though: Exploding Pie (though I'm sure this already exists).
Applejack breaks the 4th wall, etc.