Like their bigger counterparts originally were, they are invisible to humans and such, including Potterverse wizards. For those who can see them, they appear as Yorkshire terriers, roughly the size of ordinary terriers and sometimes with small bows between the ears.
At the time mini-hellhounds were first reported, hellhounds had yet to be seen on Supernatural. They always remained invisible to the audience and characters. A later episode revealed hellhounds to look like fairly standard spectral monster-wolves. It is currently unknown whether mini-hellhounds have changed appearance to match their canon counterpart, or if they have remained the same. Either way, the Legal Department is probably involved somehow.
Hellhounds are servants of demons in Supernatural, tracking down people who have sold their souls, killing them, and dragging the soul to Hell. They remain invisible, except to those close to death or when looking through glass scorched by holy fire. Despite their, uh, supernatural strength and speed, they can be warded off with salt, goofer dust or devil's shoestring, or even flat-out killed by salt, iron nails or explosions. Certain other, special artifacts can also hurt them, like an angel's sword or the Winchesters' special Colt gun.
- Sylvainia Bridge