Seu animago não terá uma pata mettalica
Olhando os escritos de Rowling sobre Animagi,
An Animagus has a great potential advantage in the spheres of espionage and crime. For this reason, an Animagus Registry exists on which all Animagi are expected to log their personal details and the precise appearance of their transformed self. It is usually the case that distinctive markings or disabilities belonging to the human body will transfer to the animal self. Failure to enter oneself onto the Registry may result in a stretch in Azkaban. When the process of becoming an Animagus goes wrong, it often goes seriously wrong. Impatience with the long and complicated process is generally at the root of such disasters, which usually take the form of horrible half-human, half-animal mutations. There is no known cure for such mistakes and those who make them are often forced to live out their days in their pitiable condition, being unable to become fully animal or fully human.
Pottermore - Animagi (behind paywall)
A nova mão metálica não é uma "marca distintiva ou incapacidade pertencente ao corpo humano ". É uma criação de Voldemort da qual Pedro claramente não tem controle.
Without pausing to think, Harry tried to drag back the hand, but there was no stopping it. The silver tool that Voldemort had given his most cowardly servant had turned upon its disarmed and useless owner; Pettigrew was reaping his reward for his hesitation, his moment of pity; he was being strangled before their eyes.
Harry Potter and the Deathly Hallows - Chapter 23 (text available for free on Pottermore)
Como tal, é provavelmente seguro assumir que não seria transportado para a sua forma animaga.
Além disso, temos um precedente canônico para a forma animaga não mudar após a primeira aparição.
The animal into which one turns, if an Animagus, seems always to be that which becomes the Patronus. There is no known instance of the Animagus form changing to match the Patronus if the latter changes, but the Animagus who can also produce a Patronus is highly unusual and no study has ever been done on sufficient numbers to draw firm conclusions.
Pottermore - Animagi (behind paywall)
Embora isso esteja se referindo a um tipo muito maior de mudança, e o Animago de Peter mudou quando ele cortou o dedo.