Doesn’t sound clumsy enough…  
really it should be ETB -> something crashes (return to hand? tap something?)
 
 
@Neo987  
er, i hate to be a nazi here, but  
>can’t attack  
Defender, bro  
>Play this ability as an instant. This ability cannot be used if Derpy Hooves does not have flying  
Tap abilities are already instant speed. instead of that last sentence, just say “Derpy hooves loses this ability”
 
but the point is, something should blow up. lol

A CITP ability like that screams to be abused by Astral Slide.

@B2C  
Because we needed American muffins…I think.

Derpy seems more like this to me:
 
Cost: 1WW  
Creature - Legendary Pegasus Force of Nature  
0/2  
Flying, Derpy Hooves cannot attack  
T: Target creature or player takes damage equal to Derpy Hooves’ toughness. She takes half damage equal to half her toughness and loses flying until end of turn. Play this ability as an instant. This ability cannot be used if Derpy Hooves does not have flying.  
T: Destroy a random artifact. Derpy Hooves takes damage equal to the artifact’s converted mana cost and loses flying until end of turn. Play this ability as an instant. This ability cannot be used if Derpy Hooves does not have flying.  
Sacrifice an artifact: Deal damage to a randomly selected creature equal to the sacrificed artifact’s converted mana cost. This ability cannot be used if Derpy Hooves does not have flying.  
Sacrifice a Muffin: Derpy Hooves gains flying and untaps if tapped.  
Any time a Muffin enters play, the controller of the Muffin gains control of Derpy Hooves.
 
“I just don’t know what went wrong!” -Derpy Hooves

@B2C  
Why not?

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when Derpy hits the opponet, flip a coin.  
if heads, target takes double damage.  
if tails, tap a land for two turns or discard 1 card from your hand.

All players draw the top card of an opponent’s deck?

@Tim Timsen
 
That’s a little excessive.

@Vree  
“Put your whole deck on the graveyard.”

Unless it causes trouble for its player too it’s not a real Derpy.