First, you need to decide how the horse element fits into the Pokemon world. Is it a new Pokemon species? Or perhaps a form of an existing Pokemon? Then, think about the 'lemon' aspect. It could be related to the plot, like a quest for a rare lemon - flavored Pokemon treat. Start with a strong outline that includes the main characters, the setting (maybe a lemon - grove - filled region), and the conflict. For example, the horse - Pokemon might be in danger and the hero has to use their knowledge of lemon - based Pokemon powers to save it.
To write this kind of fanfic, you should start by researching both Pokemon and horses. Look at the characteristics of horses like their speed, strength, and grace, and then figure out how to translate those into Pokemon - like abilities. For the lemon part, consider using it as a motif. You could have lemon - colored Pokemon or ones that are attracted to lemon - scented things. Build your story around a central idea, such as a competition where the horse - like Pokemon has an advantage due to its lemon - related power. Write in a way that makes the reader care about the characters and their journey.