One of the themes of the story is that even though you want something badly, it might not turn out to be as good as you thought.
Most children have to struggle with gender identity a bit as they grow up. The narrator of this story is no different. She identifies more with her father at first. She lives on a farm, most likely in rural Canada based on the author’s background having grown up there. The little girl seems to enjoy being outside and doing manly chores, rather than being inside. For this reason, she relates more to her mother than her father.
She was plotting now to get me to stay in the house more, although she knew I hated it (because she knew I hated it) and keep me from working for my father.
The narrator does not want to be ladylike, and finds house chores boring. The house is stifling to her, and she would rather do even the bloody chores of the farm.
One of those chores is killing the old horses that her father buys as meat to feed the foxes they raise on the farm. Until she put in the position to make a choice, she tries not to think about it. One day, the horse gets away and runs straight for her—and she opens the gate to set it free.
She does not regret being on Flora’s side instead of her father’s. She understands that she did “the only thing” she could. In the end, the horse is caught and butchered. The narrator learns that the roles boys and girls are more complicated than she thought. She can have an opinion without being entirely on either side.
Although she wanted nothing more than to do the men’s work and be on her father’s side, she learns that this is not possible. She did not have the heart to stop the horse, thus she is a “girl” and not a boy. Her father dismisses her, and she is condemned to the life of a lady.