I guess that depends on how broad your definition of "similar" is. If you take out the true story/serum run part, what you're left with is the story of an insecure wolf dog who falls in love with a girl at a distance, but because he thinks so little of himself and because nobody in town likes him, he doesn't think he has a chance with her. But when the opportunity arises to make a difference on a large scale, he rises to the challenge and in the process gets the girl.
That's overly simplified obviously, but that is the core of what Balto is when you take out the serum run. Is that an original story? Well... yes and no. It's original in the sense that the main character is an outcast because he is half wolf and half husky, and that brings its own challenges, but it's not very original in a larger sense, namely the hero is an insecure guy who comes out of his shell, does a great thing and then gets the girl. History is littered with that story told in hundreds of different ways.

Want a really far fetched example? Revenge of the Nerds. The underlying plot is pretty similar, believe it or not. If you follow the main character, he's an underappreciated outcast, reviled by nearly everybody, then he does a great thing, and in the end he gets the girl. Sound familiar? Or for a more recent, more relevant example, Alpha and Omega. Humphrey is an omega, the bottom of the pack, and while Kate liked him as a friend she saw him as a lower class and not really worth her time in any serious way. But then Humphrey proved what a great guy he was, helped prevent a war, helped save Kate's father and the other pack's leader, and got the girl.
Same story, different characters and setting. Want a video game example? Super Mario Brothers. Oh no, the princess has been kidnapped! But the lowly Italian plumber overcomes a series of obstacles and challenges, fending off death at every turn, eating mushrooms, hitting his head on lots of things, until he saves the princess.

Or Legend of Zelda, same thing. Link's princess is always in another castle too.