The Celtic Twilight is a little book, first published in 1893 and expanded a few times before its final form, Mytholgies, published posthumously in 1959. Only twenty years posthumously. It's hard to think of Yeats living until the start of World War II, but there's the undeniable entry on the lawyer page of my edition of Celtic Twilight: 1865-1939. When I went to London in 2006, I went well out of my way to get to the British Library. I had gone to see the Beowulf manuscript, cause that's the kind of nerd I am. I also listened to a recording of Yeats reading his own work. Blew my mind. But today I discovered that there's a clip on youtube of him reading his stuff. Transcribed here. Geez.
So, Yeats included today's story in the 1902 edition of Twilight, but he gives no specific source beyond "a friend." The first-person narrator of the story is Yeats himself. When scholars talk about The Celtic Twilight (here's an example, if you've got access), they tend to ignore the swine of the gods. The book as a whole devotes lots of attention to the fairies of Irish folklore, the witching hour (or twilight, if you will) when this world and the next--always close together in Ireland, Yeats thought--are especially tight. Tight enough that a child or a young bride may be abducted and taken to the fairy lands. Enough of this; let's get to the pig...
The Swine of the Gods
"The Swine of the Gods" is so close to the "Vanishing Hitchhiker" that I can't stand it. If only the swine had been in a picture hanging on a tavern wall in the village... If I'm ever called upon to tell this story, that's how it will end. I tried to find images of an old, thin-legged Irish pig, but nothing of interest showed up. A google image search for "swine of the gods" isn't nearly as interesting as you think it'd be.