This was a fun arcade game!
I really liked the look of the game. I LOVE a black skybox with retro graphics like this, it feels delightfully spooky. Once I got the hang of the controls, it was also fun to try and beat my high scores. And I really enjoyed seeing the rolling boulders crush the other enemies :P
I think the game would benefit from some music, but I *do* like the clopping of the horse and the other sound effects.
As of right now, I think this game is perfectly solid, but if you ever wanted to expand it, I could totally see this game having cool powerups, or upgrades, or new areas to explore as you get further from the start. I'm kind of just picturing the mechanics of jetpack joyride in my head, but imagining this game on top. As it is, though, it's a lot of fun. Good job!
I played the game a bit on stream here, at around 1:47:30