The story that most reminds me of this plot line appears, interestingly enough, in The Little House on the Highland series. These are stories about Laura Ingalls Wilder's great-grandmother, Martha, who grew up in Scotland. In a tale that little Martha supposedly heard as a child, a girl is supposed to marry a man she doesn't love at the order of her father. She runs off to the church in despair and overhears two mice talking about her. One says to the other that what she really needs is such and such a fix. She takes their words to heart and performs the spell, then she goes home to bed. The next morning, her wedding day, the detested suitor arrives. She appears, to the surprise of all, with the head of dog. The suitor begins to rail at her father for cheating him and says he won't have her. Meanwhile the man she loves arrives and he takes her in his arms despite her dog's head. Her head turns back to normal and she weds the man she loves and who truly loves her.
Like Brave, the spell worked an unexpected animal transformation but in the end, the desired outcome arrives. It just arrives in a way no one thought it would, via their reactions to the transformation.
Picture from Karen's Whimsey