This is a Ci Annwn (plural Cwn Annwn), a mythological Welsh hunting dog. These beasts were supposedly owned either by Arawn or Gwyn ap Nudd, both of whom were kings of the otherworldly realm of Annwn.

Like all good myths, there's a good bit of variety in the stories. Although Annwn is a land of eternal youth and plenty, these dogs were dubbed 'hounds of Hell' by Christians. To be fair, they do sound quite scary. According to folklore, their growling sounds loudest when they are far away and gets quieter as they approach. You don't really want them to do that because it's a portent of death. The Celts associated the colour red with death, hence the red ears. White was considered a supernatural colour, and is often attributed to creatures from beyond our world. They are also associated with the Wild Hunt, so I imagine other European countries have similar beasts.

I mainly based the appearance on the Welsh Hound because the breed has existed for a very long time and seemed plausibly like it could be similar to what ancient people would've imagined. I also thought it gave it more character than just drawing the meanest looking dog I could think of. I did, however, also look at wolves and based the running pose mostly on them.

Thanks to SupaStixdude for the crits about the light trail from the eye. Here's how it looks without it:

Drawn for the PJ Bestiary Collab using Morganne's Eris18 palette.