We all know how badly Ron performed magic during the 'Chamber of Secrets', especially since Ron broke his wand crashing into the Whomping Willow with Harry. But during their Transfiguration lesson, when they were practicing the spell, "Vera Verto", what if Ron's wand successfully performed the spell, but the spell only partially worked on his rat Scabbers, whom Ron had obtained from his third eldest brother Percy. As many who have read and/or watched the Harry Potter series may know, Scabbers is the mass murderer Peter Pettigrew, a.k.a Wormtail.
Since Wormtail is an Animagius, he was able to transform from a rat to a human and back again at will. As Ron cast "Vera Verto" (Animal to Goblet spell), the spell may not have worked completely on Scabbers/Wormtail, because there was already magic at work within that being.
I mean, there could be many ways in which this spell-casting could have gone.
- It could have been Ron's wand playing up all along, due to it been broken, and it not truly belonging to him, as he received it second hand from his second eldest brother Charlie.
- It may have been the fact of Ron's wand being faulty from being broken, and Scabbers/Wormtail being the transpecies he was.
- My original thoughts of the whole mis-transfiguration being entirely on Scabbers/Wormtail's part, as discussed above.
There may be a few more theories out there, but none of what I can think of. If you have any, please let me know in the comments below.