Cadogan

From Hpwiki

Sir Cadogan is a knight in one of the magical portraits placed throughout Hogwarts. He does not really have any purpose other than to chase students by running through portraits and shouting challenges at them. He is short, somewhat clumsy, and was assigned to replace the Fat Lady in Harry's third year where he allowed Sirius Black to enter the tower with a stolen password. He has a fat little pony which gallops behind him wherever he goes.