According to Wikipedia:
Udo of Aachen (c.1200–1270) is a fictional monk, a
creation of British technical writer
Ray Girvan, who introduced him in an April Fool's hoax
article in 1999. According to the article, Udo was an
illustrator and theologian who discovered the Mandelbrot
set some 700 years before Benoît Mandelbrot.
Additional details of the hoax include the rediscovery of
Udo's works by the also-fictional Bob Schipke, a Harvard
mathematician, who supposedly saw a picture of the
Mandelbrot set in an illumination for a 13th-century carol.
Girvan also attributed Udo as a mystic and poet whose
poetry was set to music by Carl Orff with the haunting O
Fortuna in Carmina Burana.