Based on the episode where Pinkie Pie is accepted as part of the Apple family, most notably the song that goes with it. The card's info says:
RB10-EN067:
Apples to the Core
Normal Spell
Target 1 "Apple" monster or "My Little Pony Younger Granny Smith" you control; it gains ATK equal to the combined ATK of all other "Apple" monsters or "My Little Pony Younger Granny Smith", but only that target can attack this turn, also halve any Battle Damage your opponent takes from attacks involving it. If you also control "My Little Pony Energetic Pinkie Pie, toss a coin, then apply this effect first if the result is Heads:
- "My Little Pony Energetic Pinkie Pie" becomes an "Apple" monster.
Rare