For the My Little Pokemon story, the ponies all have certain Pokemon that match their personality or mesh well with them and have some chemistry that can bounce off. Originally I would've gone for the simple route like Celestia having Solrock and Luna having Lunatone, but I decided to choose Pokemon that they would have not based on their Cutie Mark, but their character traits. Even for the background characters. There are some Pokemon that are mostly chosen based on their Cutie Mark like