Trevor

From Hpwiki

Trevor is Neville Longbottom's toad and is notorious for escaping his owner whenever given the chance. Trevor has not revealed (in the books at least) any magical power.

Trevor could be an Animagus. This could be supported by his constant escape attempts (a la Scabbers). However, Trevor tries to escape a lot less frequently than Scabbers, and does not try other measures such as faking his death.