Another theme could be acceptance. Harry has to accept his new form, and so do the people around him. His friends, like Ron and Hermione, may struggle at first to see him as the same Harry, and the story could explore how they all come to terms with this big change in Harry's nature.
Survival would also be a theme. In the wizarding world, being a deer might make him more vulnerable in some ways. He has to learn how to survive, whether it's finding food, avoiding hunters (both magical and non - magical), and dealing with the elements. And his friends might have to help him in new ways to ensure his survival.
One theme could be identity. Harry has to figure out who he is now that he's a deer. He's no longer the typical human - wizard, and this new form challenges his sense of self.
I'm not sure of a specific one titled exactly like that, but in general, in such fanfictions, Harry's deer form could lead to some really interesting plot twists. For example, his speed and agility as a deer could be used in magical battles or in escaping from danger. He might also form unique bonds with magical creatures that are more accepting of his new form.
One possible storyline could be Harry's struggle for freedom. He might try to find ways to escape the slavery system, perhaps by befriending some sympathetic characters within the slave - holding society. Another could be his discovery of hidden powers or skills that give him an edge in his fight for liberation.
Another theme could be transformation. Maybe the deer man is a result of some kind of transformation, either magical or spiritual. This could explore ideas of change and what it means to be different. Also, the theme of friendship or kinship could be present. If the deer man is part of a deer community, it could show how different beings can form strong bonds.
Change could be a theme as well. The arrival of the first deer in an area might bring changes to the local ecosystem. For example, the deer could start eating certain plants, which then affects the growth of those plants and the animals that depend on them. It could also be about the change in human perception of deer over time, from just a source of food to a symbol of grace and wilderness.
One possible theme could be revenge. If a character is born in blood, perhaps they seek vengeance for the circumstances of their birth. Another theme might be identity, as they struggle to come to terms with their origin in blood.
Well, without having read it, it could be that 'harry is a deer fanfiction' is a story where Harry is transformed into a deer or has deer - like qualities. It could also be a story set in a unique universe where deer play a major role and Harry is involved in their world in some way, perhaps in a quest related to the deer.
Themes in the 'Echogee the little blue deer' story might include acceptance. The other animals in the story may need to accept Echogee for who it is, with its blue color. Also, adventure could be a theme. Echogee might go on various adventures in the forest, facing challenges and learning from them. And there could be a theme of growth, as Echogee experiences different things and becomes a more developed character throughout the story.
One possible theme is the search for inner peace. In the Harry Potter world, characters often face great turmoil, and by introducing Buddhist concepts, they could seek inner peace through meditation or self - realization, like Harry might find a new way to deal with his PTSD from all his encounters with Voldemort.
One possible theme could be discipline. In the fanfiction, Snape might see Harry as needing strict guidance and uses the whipping as an extreme form of discipline, perhaps to teach Harry a lesson about the dangerous world of magic he's in. Another theme could be power dynamics. Snape has a certain amount of power as a teacher, and the act of whipping Harry could be a way to show how that power can be misused or used for what Snape believes is Harry's own good.
Themes could include the blurred lines between good and evil. If Harry is a fan of Voldemort, it challenges the traditional view of their roles. Another theme might be the exploration of dark magic. Since Voldemort is associated with it, Harry's fascination could lead to a deeper look into the nature of dark magic.