Typically seen as a man wearing a dog-shaped mask, Perro is a lot bigger than he lets on. Get him in the mud or snow, his footprints leave massive pawprints in their wake. And that's because his true form is that of a massive while spectral hound. The picture provided details both forms he takes.
Perro is some mix between your traditional hellhound and a Cwn Annwn (fairy dog).
Cwn Annwn (`Hounds of Annwn or ‘Hounds of Hell’) are Welsh Hounds of the underworld, red eared, white fairy dogs of the Wild Hunt. The Hunt is lead by Arawn, or sometimes by Gwynn Ap Nudd who is both the King of the Fairies and the King of the Dead. It rides out on wild and stormy nights to pursue the souls of the newly dead. To hear it is an omen of death. The call of the dogs grows quieter the nearer they come, sounding ‘like that of small beetles’, but in the distance their cry is full of lamentation ‘like that of a bloodhound.’ They are called Cwnwydir (‘Sky-dogs’) since their eerie cries are heard from the air, and Cwn bendith eu Meiriunau (‘Dogs of the Fairies’).
tldr; Perro is a hellhound in disguise that likes to hunt down naughty boys and girls and send them to hell, often via snarfing them up. Alive.