@Kryptchild  
I disagree, :http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=426850

Honestly, with art like this, who wouldn’t want to make a card?
 
No comments on card structure, as anything I would say has been said. I still think everyone should download Magic Set Editor, but that’s just me.

Here’s the thing: Giving all your creatures prowess makes for a very messy stack, since every noncreature spell you cast will cause a trigger for every creature you control. That’s why Soulblade Djinn doesn’t just say “Creatures you control have prowess” despite being in the set that made its evergreen status official. You may want to go with “Whenever you cast a noncreature spell, Ponies you control get +1/+1 until end of turn.”
 
Also, I’m not sure if you’re aware, but Luna’s third ability won’t trigger prowess, since you aren’t casting a spell, only copying one.
 
Also also, “You or another Pony creature you control” is perfectly copacetic. See Palisade Giant. After all, players aren’t permanents.

@Kryptchild  
I guess I could make it say “other Pony and Alicorn creatures” to still give Tia’s card the pump.

@MordekaiserHueHueHue  
Not without making the text even longer, such is the problem of designing a card with a lot of abilities. The closest aproximation I’ve found is looking at Sigarda, Heron’s Grace whom gives you and humans you control hexproof, she excludes herself by not being human. Maybe take “Pony” out of Her creature type (far as I’m aware Wizards never tacks more than 2 creature types on a creature anyway)

@Kryptchild  
Thanks for the help! Slight nitpick of my own, though: if it read “you or a pony creature,” she’d be able to target herself, which is what I want to avoid. Any suggestions on how I can clean up the language a bit while still not targeting her?

I like it! Always a fan of Jeskai, though a few language nitpicks if you don’t mind me indulging myself.