Here's why Sirius is called Padfoot:
* Animagi: Sirius is an Animagus, meaning he can transform into an animal at will.
* Black Dog: Sirius's Animagus form is a large black dog.
* Nickname: The name "Padfoot" was a playful nickname given to him by his friends, likely referring to his black dog form.
So, "Padfoot" is just another name for Sirius Black.