Yeats was also a major participant in the Hermetic Order of the Golden Dawn. He officially joined in 1890, but may have been part of the Order since its inception. Unlike other occult-leaning poets like William Blake, he does not seem to have allowed his studies of the arcane to influence his writing much. (Although the apocalyptic musings of "The Second Coming" would probably have been appreciated by erstwhile member and self-styled Great Beast Aleister Crowley.)

It's unclear whether this is connected to his Golden Dawn experiences, but Yeats seems to have had some fairly strange ideas about health -- there is a (somewhat apocryphal) story told about the lengths to which he was willing to go for a salutary effect. In his old age, Yeats had monkey testicles implanted in his own scrotum in the hopes that they would prolong his life and make him more virile. This earned him a reputation as Ireland's "Gland Old Man."