To what extent may we say that the narrator and Flora were similar?

The narrator and the horse named Flora share a few traits but are more different than they are similar. Both are female, and both seem high-spirited. The narrator is young during the period of the story, while Flora is old. The narrator is human; Flora is a horse. This distinction is particularly significant because the human character has agency, but Flora is a domesticated animal who can make almost no life choices for herself.
It is the recognition of this difference between them that prompts the girl to open the gate wide. Because she knows that the mare's fate is sealed, she seems giddy with the power to enable Flora to have one last glorious run before she dies and to defy her father with a big, obvious gesture.

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.