Sirius Black--GIANT BLACK DOG
Peter Pettigrew--RAT SCABBERS
four either never registered before their death or are currently living and unregistered (McGonagall is the only mentioned registered Animagus).
Each Animagus has a specific animal form, and cannot transform into any other animal. The animal cannot be chosen: it is uniquely suited to that individual's personality, like the Patronus Charm, and in some cases the Animagus will change into the same animal used in the person's Patronus charm (as shown by McGonagall's spectacled cat Patronus in Deathly Hallows). Similarly, when an Animagus transforms it is always into the exact same animal (i.e. same markings, same colours, etc.). When an Animagus registers, they must record all the defining physical traits of their animal form so that the Ministry can identify them.