A theme might be the journey of acceptance. Since they are such different creatures, the fanfic could focus on how they come to accept each other. Maybe the dragon has to learn to see the value in the donkey's simplicity, and the donkey has to overcome its fear of the dragon's power. There could also be themes of friendship or even an unlikely alliance, where they team up to face a common threat despite their differences.
I'm not aware of specific 'donkey x dragon fanfic' stories. However, you could try searching on some fanfiction platforms like Archive of Our Own. They have a vast collection of fan - created stories, and you might be able to find something related to this rather unique pairing there.
Friendship is a very common theme. Just like in the movie, fanfictions often explore how the unlikely friendship between the dragon and donkey grows and deepens. For example, they might face challenges together and through that, their bond becomes stronger.
Friendship is a very common theme. Their unique bond, where Shrek is the big, gruff ogre and Donkey is the chatty sidekick, is often explored in different ways. Another theme is adventure. Since they often go on adventures in the original movies, fanfics might continue those adventures or create new ones for them. Also, acceptance is there, as Shrek has to accept Donkey's presence and Donkey has to accept Shrek's ogre - ish ways.
One popular donkey and dragon fanfiction could be 'The Adventures of Donkey and the Dragon'. It often features their unlikely friendship and the challenges they face in a magical world.
Donkey is a top pick. He has so many funny lines and his friendship with Shrek is a big part of what makes the Shrek movies great. In fanfiction, he can be put into all sorts of crazy situations. The dragon is also quite popular. Her unique look and her relationship with Donkey add a lot of interesting elements. Sometimes, the dragon is written as a more gentle and motherly figure, which is different from her initial appearance in the movie.