Hellhound

Hellhounds are a kind of faerie, found in the Nevernever. They are mentioned in Grave Peril

Description
Hellhounds are dog-like creatures, being described as "great shapes, like mastiffs built from shadow and soot". (chapter?) Otherwise, they seem to share a similar physiology with actual dogs.

Like normal dogs, Hellhounds have the ability to track down scents, though they do so remarkably well in the Nevernever.

Like all other faerie creatures, hellhounds are extremely vulnerable to cold iron.

Grave Peril
They are used by Leanansidhe to hunt down Harry Dresden.