In the fourth branch of the Mabinogion, Gwydion and company move through the areas surrounding the court of Dryfed after receiving pigs from Pryderi:
That night they made it as far as the uplands of Ceredigion - and the place [where they stopped] is still called 'Mochdref' because of that. The next day they went on their way [and] they came through Elenid. That night they were between Keri and Arwystli, in a township that is also called 'Mochtref' for that reason. And thense they went onwards, and that night they came to a commote in Powys which is likewise called 'Mochnant' after that incident, and they were there for the night. And from there they went on as far as the cantref of Rhos, and there they were that night in a town still known as 'Mochtref'.
"Mochdref" literally translates to "Pig town", and "Mochnant" literally translates to "Pig valley".
The same thing later happens at a town thereafter known as Creuwyryon.
What were the original names of the four places thereafter known as Mochdref, Mochtref, Mochnant and Mochtref?
It appears that the name "Mochnant" has survived, indicating that these places may not been simply figments of a tale, but real towns.