Considering that any human being that comes to the Equestria Dimension automatically becomes a pony, it is safe to assume that those are the actual Rick and Morty.

Bonus points to the fact that going to a parallel universe to ride a famous roller coaster that's about to be decommissioned is something Rick would do, and he's a known roller coaster enthusiast (i.e. the Twirly-Whirly).

Morty: aw man Rick did we turn in to horses?
Rick: First of we are ponies not horses, those are two different animals. Second, often the times the portal leads to a universe with different laws of reality then our own and once in those universes our bodies morph to those laws. In this case everything humanoid in an equine, skin tone is replaced with pastels rainbows, magic exist, and a you get a tramp stamp on you butt of whatever it is you going to do for the rest of your life.