The Hell Mastiff is a quadrupedal creature shaped roughly like a dog, with dark grey skin, a coat of spines over its back, red eyes, large claws on its feet and spines on the elbows of its forelimbs and hindlimbs. The Hell Mastiff can project its spines in any direction and uses them to attack its victims.
In "The Shadow Phoenix," Darkar sends the Hell Mastiff, the Tar Monster, and the Brute Monster to recapture the pixies and eliminate Aisha. The monster confronts Aisha in one of the fortress' rooms and shoots its quills at her, missing her. It tries once more but ultimately ends up a victim of the Tar Monster's corrosive saliva and the Tar Monster a victim of the Hell Mastiff's sharp quills.
In "Princess Amentia," the Hell Mastiff and the Tar Monster appear in Downland caves prompting Bloom, Aisha, and Sky to run from them. Sky is knocked down by the Tar Monster in an attempt to fight it. Upon the monsters approaching her, Aisha throws the Seeds of Light down to stop them, but they cannot take root without light. Just then, Stella enters the cave and attacks the monsters. She then emits enough light for the seeds to take root and disintegrate the monsters.
#18: The Guardian of Dreams
Winx Bloomix Quest
Statues of it appear in the Hall of Enchantments. It is also a boss battle.
- Its name is derived from ”Hell,” which in many religious and folkloric traditions, is a place or state of torment and punishment in an afterlife.
- The appearance of the Hell Mastiff is loosely modeled on that of Anguirus, a kaiju monster who appears in the Godzilla franchise. Unlike Anguirus, however, the Hell Mastiff has a shorter snout and no tail.
- It is called Infernal Mastiff in the 4Kids dub.
- It is called Prickle Monster in Winx Bloomix Quest.
- In a 4Kids digital stamp collection, it is named "Prickle".