Padfoot
From UnknowableWiki
Padfoot is the nickname of Sirius Black, one of the four Marauders. It is given to him during his Hogwarts days due to the fact that his Animagus form is a large black dog (PA18).
James Potter refers to Sirius as "Padfoot" before the two students start duelling against Snape near the Great Lake, after their OWLs (OP28).
Lily Potter addresses a letter thanking Sirius for Harry's birthday present to Padfoot (DH10).
The Marauder's Map is signed with the four Marauders' nicknames:
When the Map communicates with people who do not know the password to open it, statements by all four Marauders appear on the blank parchment; each of them is signed by one of them. Severus Snape attempts to force the Map to reveal its secrets to him after apprehending it one night; Mr. Padfoot expresses his surprise that such an idiot became a teacher (PA14).
Fred and George Weasley, who find the Map in their first year at school, fondly refer to the its creators—whose real names they do not know—with gratitude, feeling that they owe much of their success to the Map (PA10, PA13).
Remus Lupin refers to Sirius as Padfoot shortly after the two old friends meet for the first time in twelve years, inside the Shrieking Shack (PA19).
Severus Snape is well acquainted enough with the Marauders' nicknames that he seems to recognise them when reading the Map's insults (PA14). Likewise, when Harry Potter finds himself cornered by Dolores Umbridge and desperate to warn the Order of the Phoenix that Sirius is in danger, Harry shouts to Snape, "He's got Padfoot! He's got Padfoot at the place where it's hidden!" Severus immediately understands the cryptic message (OP32, OP37).
