To become an animagus takes a great deal of work and magical ability, but that would hardly be an obstacle for the Greatest Wizard of the Age. Additionally, transfiguration was his specialty, as he was the professor before McGonagall. Obviously, he wasn’t registered with the ministry, but most of the animargi in the books aren’t. It would seem in character for him to accomplish becoming an animagus.

Further, is it possible to become different types of animals when you transform? This entire question hinges on Lupin telling Harry at the end of PoA “Yes, your father was always a stag when he transformed.” The use of the word “always” here implies that some people are able to become different animals.

I’m curious to hear other theories on animargi!