What of Curumo?

[The Might]

A very, very interesting theory.

I was curious to learn more so I of course did what all others do - Wikipedia.
But listen to this interesting part:

In this narrative, Kentigern meets in a deserted place with the naked, hairy madman Lailoken, also called Merlynum or "Merlin", who declares that he has been condemned for his sins to wander in the company of beasts.

Also:

C. S. Lewis used the figure of Merlin Ambrosius in his 1946 novel That Hideous Strength, the third book in the Space Trilogy. In it, Merlin has supposedly lain asleep for centuries to be awakened for the battle against the materialistic agents of the devil, able to consort with the angelic powers because he came from a time when sorcery was not yet a corrupt art. Lewis's character of Ransom has apparently inherited the title of Pendragon from the Arthurian tradition. Merlin also mentions "Numinor," a nod to J. R. R. Tolkien's Númenor.

Who knows if the Professor wasn't somehow influenced by this and was inspired a bit when he created Radagast.

Just a theory, but a good one I think.