If you didn't pull the Desert Thorn out of Dorothy earlier, I believe you will be unable to ride Dorothy.

I am not sure. I played the game twice and never saw this. But - based on the symmetrical design of the game and its branching paths, I am guessing that if you find Bruce in the Settlement marketplace, he will give you something you can use.

If you know the answer to this, let me know. @indiegames_muso on Twitter.