The Mabinogion is an ancient collection of Welsh folk tales that fairly obviously consists of a body of pagan myth that acquired a Christian veneer during the middle ages.
This is relevant because one of the legends tells how the mortal hero Pwll, Prince of Dyfed does a service for Arawn, lord of Annwn (a fairly obvious analogue of Pluto or Hades). Arawn's gratitude for this takes the form of a gift of the first pigs to be seen in Wales, and this divine origin seems to have the result of making pigs seem "special" for the rest of the cycle (a later story features a war between Dyfed and the northern kingdom of Gwynedd for possession of the pigs).
So to get pigs to continue to be seen as sacred animals you need either a surviving Welsh pagan tradition (borderline ASB, IMHO) or (rather more likely) for the pagan tradition to survive long enough and be strong enough for the idea of pigs as sacred to be syncretised into at least the local version of Christianity.
Dunno how fundamentally likely either option is, but at least pigs were sacred once
