One possible theme could be friendship. The story might show how a human and a hippo form an unlikely bond. For example, a human might rescue a hippo calf and then they grow up together in a unique way.
Another common theme is survival. Mr Hippo has to find food and water, which are essential for his survival in the wild. So, the stories might show how he overcomes difficulties to get these resources.
There are not many mainstream hippo and human fanfictions. However, you can try to search on some specialized fanfiction platforms like Archive of Our Own. Sometimes, independent authors create unique stories involving such unusual pairings.
One common theme is friendship. The relationship between the human and Senketsu often develops into a strong bond of friendship. Another theme is growth. Both the human and Senketsu usually grow as characters throughout the story, learning from each other. Also, the theme of facing challenges together is quite prevalent as they encounter various obstacles in their adventures.
Self - discovery is another theme. Through their relationship with Toothless, the human often discovers new things about themselves, like hidden courage or a new perspective on the world. And for Toothless, his interactions with the human can also lead to self - discovery, perhaps learning about the value of a different kind of relationship compared to those with other dragons.
One common theme is friendship. Like in many stories, the dragon and the human start as strangers but gradually become friends and rely on each other. For example, in 'A Dragon's Friend', the human helps the dragon when it's trapped and in return the dragon protects the human from danger.
Often, there's a theme of hidden powers or abilities. Even as a human, Jasper might have some remnants of her previous self. This could be in the form of enhanced senses or a special intuition. Fanfiction writers like to explore how she uses these to her advantage or how she tries to hide them from others in the human world.
Power growth is a very common theme. Writers often explore how the characters can become stronger in different ways compared to the original story. Another common theme is relationship development, like the romantic relationships between characters.
Another theme is cooperation. Sometimes, they realize that by working together, they can achieve greater things. In some stories, they join forces to fight against a common enemy like an ancient evil or a powerful dragon. This theme shows how they can overcome their differences and use each other's strengths. It often involves a lot of character development as they learn to trust one another.
A common theme is friendship. The human and the rabbit often become friends in these stories, like when the rabbit is a stray and the human takes it in. Another theme could be adventure. They might go on a journey through a forest or an unknown land together.
Love is a very common theme. Usually, a mermaid and a human fall in love despite their differences. Another theme is the struggle between two different worlds. The mermaid belongs to the underwater world and the human to the land world, and they have to find a way to be together.