Doom Hounds are large, hound-like monsters with a tough hide and fire breath.
A Doom Hound attacks an amusement park, killing a tiger and a ringmaster with its fire breath, before Jake and Hayley trick the Doom Hound into chasing them into a Compressor, which shrinks the Hound into a puppy.
Jake then carries the Doom Hound around with him all day, but when he takes it to Hayley's house, Cadence and her friends take the Doom Hound from Jake, exposing it to bright light. This allows the Doom Hound to revert to its normal form, and, enraged over being compressed, it begins destroying Hayley's house. However, Jake, Hayley and Etienne throw chemicals disguised as a frisbee at the Doom Hound. The Hound grabs it in its mouth and the chemicals then cause it to explode.
Cadence distracts the monsters long enough for Jake and Hayley to sneak past them to a portal. When the monsters notice, they launch an attack and chase after the three. However, the three escape through the portal, which closes behind them before the monsters can follow them through.
Doom Hounds are vicious and fierce, but they also like to chase sticks and frisbees, like ordinary dogs. If a Doom Hound is compressed into a puppy, if and when it is able to return to its normal form, it will be extremely enraged over being compressed.
Doom Hounds look like massive, dark-grey dogs with massive fangs, horns, claws, and glowing, fiery orange eyes and mouth.
Doom Hounds possess fire breath and extremely tough hides. Also, if a Doom Hound is compressed into a puppy, it can use bright light to restore itself to its normal form.
Commpressers cause Doom Hounds to shrink into puppies. It is also shown that when a Doom Hound ingests particular chemicals, it will explode and die.