looks like a miniature horse like mine, or a fallabella, or possibly a Shetland, though not likely because of the size. :)
I can just see my mini hoping in to the car along with our dogs, if we would let her. She plays with the dogs, and even sleeps in the giant house with them.
I know people that get their miniature horses to the horse show that way! The minis just jump right in like its a trailer!
Your question is how can he fit in there. Simple, he's a mini and the back seat and the passenger's seat has been removed, so plenty of room. Enters through the hatch back!