Arawn

He is the Celtic god of the otherworld, the gray cloaked lord of Annwn. It is said that he likes to take his Hounds of Hell for a run to track down lost souls. In one tale, Pwyll set his hounds on the same deer that Arawn was after. To pay for the mistake, Arawn asked Pwyll to trade places with him for a year and a day and to defeat Hafgan, Arawn's rival, at the end of that time, something Arawn had been trying to do, but had never been able to. Arawn, meanwhile, takes Pwyll's place as lord of Dyfed. Arawn and Pwyll become good friends after that. The friendship between the two realms remained long after Pwyll's death. In athoner part of the tale, Pryderi, Pwyll's son and lord of Dyfed has received a gift of otherworldly pigs from Arawn. These pigs were eventually stolen by a Venedotian magician and a trickster, Gwydion fab Don, leading to Pryderi's invasion of Gwynedd. In the ar following this, Gwydion kills Pryderi in single combat.
In Welsh folklore, the Cwn Annwn or Hounds of Annwn run across the skies in Autumn, winter and early spring.