(Has to be accepted by the general public as existing - no dragons or unicorns), what would you turn into and what would you do?
Remember people won't be able to recognise you and if you're something dangerous in a city you might be killed, or you could be preyed upon by another animal or hungry homeless person. Would you try to become a spy and turn into a bird for recon missions? But then the government would know you are a shapeshifter and might dissect you to get the secrets of how to do it. I think I would just turn into a black cat and mess with people, then I'd probably get killed by some superstitious person.
Bonus round: The longer you stay transformed the more the "animal mind" fights back and tries to take over, if you stay transformed for too long you will lose your humanity and really be that animal forever.