"Merlin's beard..."
-- Amos Diggory (GF6)
Merlin, a Charms specialist sometimes known as “The Prince of Enchanters,” is unquestionably the most famous wizard of all time (PS6, FW).
He was part of the Court of King Arthur (King Arthur once ruled the land that is now part of England). He believed that wizards should help Muggles and therefore created the Order of Merlin to support laws protecting and benefiting Muggles (FW, Pm).
Merlin was a friend of Sir Cadogan (Pm), the knight in the painting that protected Gryffindor Tower after the Fat Lady was slashed by Sirius Black.
Commentary
Notes
The Merlin of British legend is not a real person, but is based on various persons from before 1000 AD. However, since Rowling's Merlin was a Slytherin, we know that he must have lived after the school was founded circa 990AD.
According to Wikipedia, "(t)he standard depiction of the character first appears in Geoffrey of Monmouth's Historia Regum Britanniae, written c. 1136..." For the sake of argument, we'll assume that the Merlin of Rowling's world lived between the founding of Hogwarts a little before 1000 AD and Geoffrey of Monmouth's writing. Therefore we'll list the years for Merlin as the 1000s.