This is Apple Jack's torterra, Aceymac, or 'Ace' as she nicknamed him.
In the Apple Family, all members get a Turtwig when they get their cutie mark. Once they evolve into a Grotle, they plant their first apple seed onto it's back, which then sprouts into a portable apple tree when it becomes a matured Torterra. Applejack got Ace when she came back from Manehatten, and getting her cutiemark. Because she valued family, she felt Ace is part of hers, and became very close to him. Often times, they can be seen taking walks together, or her napping on his back. Granny Smith says they are so sweet to one another, that torterra grows the sweetest apples because of it.