Have you ever wondered why Peter Pettigrew never transforms into a rat after Prisoner of Azkaban? The Reddit user Chainmaillekid has — and they've come up with a pretty fascinating theory: Wormtail's silver hand prevented him from turning into a rat. According to reddit user Chainmaillekid, Voldemort deliberately hampered Wormtail's power so as to better control him. Ultimately, if Wormtail had still had the ability to transform into his ratty Animagus, he might have been able to save his own life.
Chainmaillekid points to a few specific instances where we might expect Peter Pettigrew to have transformed into a rat. One example is in Harry Potter and the Half-Blood Prince, more than a year after Voldemort first gave him the silver hand, when Wormtail is trying to eavesdrop on Snape's conversation with Bellatrix and Narcissa. Surely it would make a lot more sense for him to sneak up on them disguised as a rat? And then there's the moment the silver hand turns on him, and Pettigrew has to fight for his life. It's not like the element of fear and shock would have prevented him from escaping; after all, he was in a similarly dangerous situation in Prisoner of Azkaban and still managed to transform into a rat to escape. Nor does he need a wand; Harry had already disarmed him before that happened. So really, there's no reason that Peter Pettigrew wouldn't have turned himself into a rat — unless he couldn't.
You've got to admit, this theory makes a ton of sense. While Pettigrew was caring for Voldemort during Harry Potter and the Goblet of Fire, he hints at his ability to "disguise [him]self most effectively," so we know he still had his powers intact back then. By the end of that book, however, Voldemort no longer relies on Wormtail, and so he gives him the silver hand as a "gift" to keep his loyalty and keep him under control — a gift that destroys him the moment Wormtail hesitates over following Voldemort's orders. Yeah, Voldemort is evil, you guys.