Mayhem and Mirage Moon are prank buddies and they choose the wrong pony to prank/scare for the fall. Melrose may be shorter than an average mare, she is stronger than most stallions, and rather than getting scared her reaction is to take out the "threat".

Needless to say, they decided not to try to prank her like this in the future.

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It's been a while since I drew my next gen MLP characters, so for refresher:
Mayhem is Discord and Fluttershy's son, Mirage Moon is Trixie and Starlight's daughter and Melrose is Big Mac and Sugar Belle's oldest daughter.