The trash slug is a giant worm with the characteristics of leeches. Its body is cylindrical and tapers towards the tail. Its mouth is a jawless orifice lined with large, pointed teeth and ringed by five stubby feeding tentacles that are used to grasp food and protect the mouth when not feeding. Large eyes are spread across its back and head, and large black spines run along the back between the eyes located on the head and those on the back. Coloration is dark green across the dorsal surface and light green on its ventral side, which is divided into segments like annelid worms.
Trash Slugs are hatched from eggs approximately the size of a softball. The eggs are orange and transparent, leaving the larva clearly visible. Newly hatched trash slugs are voracious and will take full advantage of any available food source. They grow rapidly, so long as the food is plentiful. Trash slugs are omnivores, consuming anything that crosses their path, including inedible trash and debris. Anything undigested remains within the trash slug until death.
When well-fed, their bodies grow to become muscular and strong enough to ram solid wood and stone; they also become able to spit burning liquid against prey too far from their grasp. While resilient survivors, salt is highly toxic to them, as it makes them lose the mass gained.
Because trash slugs retain any indigestible material they consume, Eda views them as a treasure trove of potential merchandise for her collectibles stand. Much of the trash she found within one was clearly of human origin; everything from traffic signs and musical instruments to home appliances and nuclear waste was found inside a single carcass. As the trash slugs are encountering human trash in such quantities, this suggests they feast upon items that are found wherever there is portal energy of the Boiling Isles created by Titan's Blood, which serves as a conduit that transitions elements between the Demon Realm and the human world.
One notable trash slug, named Prince Jr., was raised by King to be one of his minions. By feeding it treats as "positive reinforcement," Prince Jr. grew to a massive size in a short period of time, and when King ran out of treats to appease the trash slug, Prince Jr. attacked its "father." Prince Jr. was subsequently defeated by being fed with salt, causing it to shrink back to a diminutive size.
- Jaws: Eda dissecting the trash slug carcass to Luz' disgust is a reference to the scene in Jaws where Brody and Hooper are dissecting a tiger shark carcass looking for evidence of a missing child and pulling out indigestible debris.
- The use of salt in defeating trash slugs is a reference to the use of salt as a real-life solution in getting rid of real-life slugs.
- However, unlike real-life slugs, which have to be sprinkled with salt on their bodies in order for the salt to work, trash slugs have to eat the salt in order for the salt to have an effect on them; and while real-life slugs die slowly from dehydration after being sprinkled with salt, trash slugs simply shrink after eating salt.