The only time I've met this name is in The Forsythe Saga by John Galsworthy. I did not know it was a form of Julian and pronounced as Julian, I thought it was pronounced as Jo-lion (like the animal). I like Jolyon but maybe others would make the same mistake as I did.
To me, this is one of the most beautiful and unique names I've ever come across. I first heard it from the Forsythe Saga, and it hasn't left my mind since.
In the novel "Endymion Spring," by Matthew Skelton, Professor Jolyon Fall at Oxford University befriends one of the main characters, Blake, and his sister, Duck (not her real name of course).