In order to earn this new pet, you'll have to complete The Real Big Race (not to be confused with The Real Race from last month's faire - the new one is on the opposite side of the docks from the previous race) within 20 tolls or less. You must use a specific vehicle as well, the Rocketeer, which is basically like a mechanical tallstrider. It's got some neat tools that come in handy during the race though.
For me, it helped to watch a video (or two...or three) before attempting it. I found the ones listed in this thread quite helpful.
Honestly though, I kept hitting a wall of roughly 21-22 seconds, and shaving off those last few to get 20 tolls or under was a pain. I don't even remember how I did it exactly, perhaps I just got lucky. Since I'm not confident in my ability to run through the course, I won't even try to attempt at a guide lol. My heart was literally about to pound out of my chest each time I got close. It was nerve wracking and I don't think I'll do it again if I don't have to lol. I'm just not good with timed content!
One thing I must say, you can tell that the Rocketeer vehicle was created specifically for The Real Big Race. It felt pretty clunky in the other version of the race. Its tools didn't feel like they fit well with the mechanics of that particular course, and I swear it almost seemed as if it was slower than the normal tallstrider.
But yay, new pet! I had tons of Mech stones just waiting to level it up to max.
Much like the Darkmoon Zeppelin, if you bring out a Lil' XT or Landro's Lil' XT near it... well let's just say he just doesn't know how to play nice. XD It's got a strange bug where it doesn't disappear immediately when destroyed by XT though, and it hovers around for a while before finally disappearing.