Can I make up a new item that reverses evolution for my fanfic?

One of my characters is a Teddiursa, and he's kind of a doctor. He's made his own form of Rare Candies, but these kind only work temporarily. So if he eats one, he can turn into an Ursaring but only but only for a limited amount of time. Can I do that?

Do whatever you want. What matters is if you have a good explanation for it.

Sure go ahead! Just make sure it fits within the rest of the tone of the story, and that you don't abuse it too much to solve everything ever, or pull it out of nowhere with no explanation part of the way through the fic. It'll be just fine if you establish it immediately, sort've as the basis of "what's different" in this story or the ONE special thing about the character. Establish it well, as early as you can, and don't overuse it; that's my suggestions!