The link between pigs and the siting of "celtic" religious houses is particularly strong.
In the 12th C Life of St. Cadog* (Translation) Cadog rests under an Apple tree surrounded by pigs before setting up his church, which he does by following the track of a white boar. St. Brynach (Translation) sets up his church where he finds a wild white sow with white piglings. Dubricius* (Translation) sets up his school on the "island" of Mochros ("Place of the Hogs") on the Wye, again, following a white sow and her piglets there. Kentigern* (Translation) has a white boar signal to the saint to follow him, leading him to a hill where he was to worship. St Kieran* (Translation) had less luck - he had to find his own churchyard, however, when he found the ground a wild boar charged him, but then instantly became tame, and aided him in building the church.
There is also a secular example of a boar acting as a guide in Welsh literature: in the Mabinogion story "Manawydan" (Translation), a white boar leads Pryderi and Manawydan into an enchanted fort. Generally, in Welsh tales, otherworldly creatures are white, usually with red ears or fronts (for example, the dogs of Annwn in Pwyll [Translation] or Arthur's mixed-colour Cattle in Cadog's Life* [Translation]), and it seems likely these white porkers fall into this category. However, the colour is morally/sociologically ambiguous and often used in jokes, possibly as an impossible colour for normal animals: for example, the bad king Maelgwn, a common foil in the saint's Lives, has a pack of white dogs and gives St Tydecho a set of white horses to be "fed with prayers" - which Tydecho subsequently feeds so well they turn golden*, while King Furbaidhe in the Life of St Kieran* (Translation) demands Kieran finds him four bald cows with red bodies and white heads as a joke.
In the same way as Culhwch was named after the swine he looked after (See Culhwch ac Olwen), one of St Patrick's followers was a swine-herd named Mochae*. He promised the saint at shorn pig every year (Translation) and at the time of writing, the pig was still being given by Mochae's church at Edrum to the church of Down (Translation). Like the Twrch Trwyth, who was once a man in Ireland (See Amr's Tomb; Llyn Liuan), this may reflect an older tradition of humanisation. Patrick himself was saved from slavery by pigs, as he found gold to buy his manumission in a hole rooted out by swine. Later, under the same circumstances he found gold to buy the land for the church of Alfind* (Translation).
Sometimes, however, other animals fulfill the same role: Patrick founded the church(?) of Telac-na-licce on the basis of the movements of a doe* (Translation), while Cadog's father Gwynllyw founded his church where there was a lucky white ox with a black spot beneath its horns, naming the place Dutelich "black-front-ox" (Translation). The white ox should, of course, call to mind the White Cow of Ireland (Summary; Well), and, more recently, the White Buffalo of Native North Americans.