Why are there no portraits of Merlin at Hogwarts? (Please note, I am referring to the Merlin from the Harry Potter series, who was alive around the 12th-16th centuries, not the Merlin of King Arthur's round table from the 5th-6th centuries.)
Merlin, in the Harry Potter series, was alive well after Hogwarts was founded and graduated from the school himself; as a Slytherin, might I add. He went on to become one of, if not the most famous and powerful wizard(s) of all time. He was also supposedly a friend of Sir Cadogan, a knight who was painted in Hogwarts.
So, if Merlin was a former student, a friend of, and therefore alive at the same time as, Sir Cadogan, and the most famous and powerful wizard of all time, how could there not be any portraits of him at Hogwarts?