It was kinda obvious. I mean, Fluttershy Rainbow Dash is the fastest Pegasus alive and why not pair Rainbow Dash with the OPPOSITE animal you can think of?
Episode 7
But still, so many fans made wild guesses she's get a turtle as a pet. I do admire though it was the turtle's loyalty that made him her pet.
Oh snap. Maybe they aren't opposites after all. Dash is supposed to be the Element of Loyalty. Tank = loyal. It all makes sense now!