Monk

A monk was a male member of a religious order. Many different religions had monks as a part of their ecclesiastical structure. Monks of whatever religion typically resided at a monastery. The Doctor had many encounters with monks over the course of his lifetime. (TV: The Abominable Snowmen, Tooth and Claw, A Good Man Goes to War, The Bells of Saint John, The Rings of Akhaten, AUDIO: The Book of Kells, COMIC: Dragon's Claw)

One Time Lord was so enamoured of monks that he adopted their title as his name to become "the Monk". He followed the examples of the Doctor and the Master in taking his name from a generic title. (TV: The Time Meddler, AUDIO: The Book of Kells)

The adherents of the Order of the Headless were known as the "headless monks" due to their appearance. They believed that the domain of faith was the heart and the domain of doubt was the head, so they chose to have themselves beheaded. At least one soldier was also volunteered at Demons Run to join the Order of the Headless. (TV: A Good Man Goes to War)

In the 12th century, Friar Tuck, member of the Merry Men (an outlaw band led by Robin Hood) was a monk or at least pretended to be one. (TV: Robot of Sherwood)

A species which invaded Earth in 2017 were known only as "the Monks". (TV: The Pyramid at the End of the World)

In 5343, King Hydroflax had monk bodyguards. (TV: The Husbands of River Song)

Behind the scenes

 * In real life, the actor Tom Baker became an initiate and trained to be a monk at the age of 15.