Well, generally Marco was the horse, as opposed to the person riding it. He didn't know a thing about horse riding. (But it couldn't be that hard, right? You just had to sit there and hold on. It's not like he was a real horse that you had to know the commands for, after all).

But before Marco could think on that too much, the scene suddenly changed. He reared slightly, startled by the new explosion of sights and sounds. <What?!> he said, blinking. <A racetrack?>

All around them were crowds cheering, and other horses being ushered into the gates. Why the hell had GLaDOS put them here?

Then, just as suddenly as they'd ended up here, half of Marco's vision was blocked out - by the blinkers that had appeared. And they weren't the only thing - in an instant, Marco had also acquired a brightly coloured saddle and a bridle.