Another theme could be loss. Maybe things didn't work out between you two, and the stories are about coming to terms with that loss and how it changed you. It could involve the pain of separation and the process of moving on.
Friendship could also be a theme. Even if it started as a romantic love, there might be elements of friendship in the stories. You might have shared a lot of laughter, supported each other through tough times, and had a bond that was more than just a romantic relationship.
One possible plotline could be that the girl loved the boy secretly, and the story revolves around her internal struggle to either confess or keep her feelings hidden.
Love and loss could be a major theme. Since it's about a woman once loved, there's likely to be a sense of what was lost in that relationship.
The theme of redemption could also be present. If the boy - turned - cow has a chance to regain his human form, it could be through acts of kindness or self - discovery. For instance, he might show unusual compassion as a cow towards other animals, which could lead to his transformation back. Also, the theme of the unexpected and the power of magic is quite common. Magic can be a force that changes the natural order, like turning a boy into a cow without warning.
One possible theme could be about self - acceptance. The young boy might initially be embarrassed in a CFNM situation but then gradually comes to accept himself as he is.
One possible theme could be self - discovery. If the boy loved exploring new places, his journey of love for that could lead to him finding out more about himself.
One possible theme could be the transience of love, like how rain doesn't last forever. The love in the story might be as fleeting as a rain shower.
Another theme might be growth. Through their interactions with the birds, the boy or boys could grow in understanding, compassion, and a sense of responsibility. For example, if they rescue a bird, they learn about taking care of another living being. This could be a step in their personal growth journey.
One theme could be misjudgment. Just like in the books, Ron is often underestimated, and in a 'not loved' fanfic, this could be explored further. Maybe the characters think he's just a sidekick and not worthy of love or deep friendship, but then discover his hidden depths.
Love is likely a major theme. It could be about different kinds of love such as romantic love between partners, the love of parents for their children, or the deep friendship love.
I think one theme would be the importance of non - human relationships. In a world where human - human relationships are often emphasized, this book might focus on how the girl's bond with animals enriches her life. There could also be a theme of innocence, as the love of a girl for animals often has an element of pure, untainted feeling.