Sirius Black. Animagical form

This is first part of the fanart project dedicated to Sirius Black. This my interpretation of his animagical form.
The idea was born after I thought: What if Sirius once argued with James and lost? What if Potter made him be a "packhorse" all day? What if the marauders realized that it could be very useful for smuggling alcohol into Hogwarts and the basics for pranks?