A runner game is always fun, simple, and generally pretty addictive, and you did a good job with making that!
A couple of things though. There's nothing wrong with having differing art styles in the same game, but in this case, everything ended up clashing a little. What I mean is that the rocks looked kind of 3D while the unicorn looked 2D, the rainbow in the background was hyper realistic while behind some cartoony mountains. Again, nothing wrong with mixing styles, but there was a lot going on here.
The main thing is the connection to the theme. I know the second unicorn is connected to you - it follows your actions after you do them - but to be honest, it doesn't really accomplish much. If the second unicorn wasn't there, you'd play the game the exact same way, you know? I don't have to do anything to prevent the second unicorn from dying except pay attention to the first one. I ended up kind of forgetting the second unicorn was there as a result. I don't think it's a bad idea, but it needed a little more to really bring it home.
Overall, great work!