"Stand and fight, you mangy cur!"
-- Sir Cadogan (PA9)
Sir Cadogan is a little knight whose picture hangs in a seventh-floor corridor near the South Tower. He is a silly fellow whose bravura outshines his common sense and who glories in quests and challenges.
He actually lived during the time of King Arthur and the Knights of the Round Table, and the other knights thought he was “hot-headed and peppery.” He was friends with Merlin (Pm). He once got into a tricky situation with a dragon-like beast called the Wyvern of Wye. It swallowed his horse, melted his visor and sword, and broke his wand. But brave Sir Cadogan found a fat gray pony and charged back towards the fierce creature, splitting its tongue with his broken wand, and causing it to explode (Pm). He kept the pony, and it’s the very one shown in his portrait at Hogwarts. When wizards say “I’ll take Cadogan’s pony” they mean to make the best of a horrible situation.
Sir Cadogan spent some time guarding the Gryffindor Common Room after Sirius Black attacked the Fat Lady (he was the only painting brave enough to take the job). He thought up ridiculous passwords and changed them on a daily basis. He challenged people to duels whenever they would try to enter or leave through the portrait hole. Sir Cadogan has a huge sword and a little fat pony (PA6, PA9) He chased after Harry when Sir Cadogan’s picture was hanging on a landing near the North Tower and the Divination classroom, but was stopped in his pursuit when he entered a picture which had a large and angry-looking wolfhound in it (OP12)
Neville Longbottom in particular had trouble with Sir Cadogan, who confused him with so many passwords that he made a list so he would remember them. Unfortunately, Sirius Black broke into the castle and used Neville’s list to convince Sir Cadogan to let him enter Gryffindor Tower, where he threatened Ron and Scabbers with a knife (PA13). Professor McGonagall was so furious she fired Sir Cadogan and sent his portrait back to the seventh floor. The Fat Lady’s returned to her rightful place, and Neville was punished with no Hogsmeade visitation, a detention, no passwords at all, and a Howler from his Gran (PA14).
- Sir Cadogan is famous among Hogwarts students and alumni. Bill Weasley, upon visiting Hogwarts after a five-year absence, specifically asked if the "picture of the mad knight" was still around (GF31).
- The brave but foolhardy little knight ran from painting to painting during the Battle of Hogwarts, shouting encouragement to the combatants (DH31).
According to the tales, Sir Cadogan had three wives -- all of whom left him -- and seventeen children.
Insane bravery, making him a good man in a tight spot (Pm)
Cadogan comes from a Welsh name "Cadwgan" (Ka-DO-gahn) meaning "glory in battle." The name is mentioned in the collection of Welsh legends known as the Maginogion.
The name is also a humorous pun because it's pronounced "ka DOG an" and he challenged students with insults like "cur" and "mongrel," and yet Sirius Black, the dog Animagus, tricked Cadogan into letting him pass by reading Neville's list of passwords. This joke arises again in Harry's fifth year when Cadogan challenges Harry to "stand and fight, you scurvy cur," then tries to follow him down the hallway through another painting, but is stopped by a "large and angry-looking wolfhound" (OP12).
Played in the Prisoner of Azkaban film by Paul Whitehouse IMDB