Every Animagus is required to register with the Ministry of Magic at the Improper Use of Magic Office. This registration includes revealing one's animal form and one's distinguishing markings. The register is available to the public. The primary reason for registration is to ensure the Animagi do not abuse their abilities, as the Ministry is able to keep track of them. - Animagus, Harry Potter Wiki

Hermione said they were only 7 registred Animagi in the twentieth century, but during the books we had meet as much as four unregistred (James Potter, Peter Pettygrew, Sirius Black, Rita Skeeter) and in the seventh book, I was left with the impression that Voldemort was one as well.

Agree:)

I do not think that they wish to register becacue it will give them an ege in things they can brake the law and get away. one thing i learned from LotR and HP is that people love power and they are willing to do just about anything to get it being an Animagus may be one of them.

never before though about that, but when you´re saying that...totally! ;D