I present you: professor Potter, Defence Against the Dark Arts teacher and Head of Gryffindor House. And Teddy Lupin, his godson and the sassiest Hufflepuff // yes good. this is much better than auror harry
But, actually, he chose a stag because it was large, powerful, and fast. It could survive a werewolf attack (assuming the worst case scenario happened). When the Marauders told Remus they were animagi.