In Alice Munro's "Boys and Girls," the narrator, a girl approaching puberty who lives on a fox farm with her family, and Flora, the horse whom she helps escape to help her avoid being shot, share a couple similarities, which may be why the narrator feels so compelled to let Flora go.
One similarity is that neither Flora nor the narrator is in control of their own destiny. The narrator is suddenly realizing that, now that she's older, more traditionally girl-like behavior is soon to be expected of her (previously she had been what is often called a "tomboy"); Flora is about to be shot because she has been deemed too old to be of use on the farm.
Another similarity they share is their defiant spirit. Flora is described as having an air of "gallantry and abandon" and is often so rambunctious that her caretakers can't enter her stall, lest they get kicked. Similarly, the narrator defies the sweet and tidy behavior expected of her by her mother and grandmother, preferring to help with the rough and dirty work on the farm.