These catalysts can be magic potions, wishes, science fiction substances, spells, or the like. I have employed all of these in my stories before.
Catalysts, though, need not be limited to items which exist in the realm of fiction. Stories exist in which transformations occur using strictly natural processes. A new job, a new relationship, or reaching a new stage in live can become the impetus for a series of changes.
The advantage of the fiction-based catalysts is that they allow for more varied transformation types and they allow for the writer to have the transformation's pace to be as is convenient.
The advantage of realistic catalysts is that because they are realistic, the possibility of them actually happening in real life exists, even if it is remote. The 'could it really happen' idea creates tension but limits the possibility of what can be written.
The pitfall of fiction is that too much of the story can be devoted to explaining how it works, or failing at that, it is not explained in a way that makes sense to the reader and just appears to be a plot device. The downside of realistic catalysts is that the change can be too subtle - or, alternatively, the change could be so significant in scope that the real life cause of them seems an inefficient and even silly explanation.
What are your favorite types of catalysts - fictitious or real-life based? What are your favorite catalysts in your favorite transformation stories?