Not saying your main point is wrong, but Black Beauty wasn't written to appeal to young girls. It is an urgent call to treat horses better, and has been called "the most influential anticruelty novel of all time."

https://en.wikipedia.org/wiki/Black_Beauty

As a young boy I certainly cried when reading that book.

And "King of the Wind".

I think that at a certain age anything that is attractive to girls can become toxic to boys, which can entrench a gender divide.