How Champ was able to get Rocky on that motorcycle that he may or may not have stolen is still a mystery. All the citizens of Ponyville know is that one minute they’re going about their day, and the next they see screaming colt clutching for dear life to a stallion who is looking like he’s having way to much fun for someone who’s never driven a motorcycle before.
And no, motorcycles are not canon in my nextgenverse but for the purpose of this scene let’s ignore that fact.