Garbage Worms are stretchy, worm-like creatures, found mostly in and around the Garbage Wastes implanted into either partially submerged spaces or terrain. They serve as an annoyance, stealing Spears from unlucky Slugcats and Scavengers.
There are a few ways to avoid losing Spears to Garbage Worms:
Waiting for the Garbage Worm to hide. Standing near one for long enough causes it to retract back into the den, providing a few seconds to get past. Since Garbage Worms ignore Spears that are on the ground, simply dropping it and scaring them away is a great method. Alternatively, holding the Spear in the hand opposite the Garbage Worm also works, since in that case the Slugcat's body can block the Garbage Worm for long enough for it to go away.
Taking it back - it is possible to retrieve the Spear while the Garbage Worm is going back into the den. While Garbage Worms also drop Spears when stunned with Rubbish, this method is fairly risky since it aggravates every Garbage Worm in the region for the rest of the cycle.
Blinding the Garbage Worms. A niche but effective method if a Hazer or Flashbang is available.
Spearing a small creature such as a Vulture Grub, then picking the creature up without dislodging the Spear. Garbage Worms don't steal Spears that are carried this way, as they are not considered as being held. This can be done with multiple Spears at once.
Jellyfish can shock them, but precision is required as they must hit the Garbage Worms right at their base.
The Gourmand's weight can knock Garbage Worms unconscious, but Gourmand's body must hit the Garbage Worm's heads.
Though Garbage Worms are stationary, they can move around the room by changing the den they come out of. Their bodies wave at an increasing frequency while observing creatures, though they often retract back into their den if one gets too close. Left unbothered, Garbage Worms stick their heads in the ground from time to time, seemingly sucking up matter which passes down through their bodies.
Despite ignoring Spears found on the ground, Garbage Worms actively attempt to steal them from creatures. They do this by stretching towards the Spear, grabbing it with their heads on contact, and quickly retreating underground. The same applies to Spears on the Hunter's or a Scavenger's back.
Though Garbage Worms cannot be damaged by attacks, hitting them with weapons such as Rubbish or Spears turns all Garbage Worms in the region hostile for the rest of the cycle, which is indicated by a change in eye color from white to red. In this state, they actively try to grab and harass the Slugcat. On land, they do this by simply flinging them with their heads, while around water they do this by holding the Slugcat underwater to drown it.
Garbage Worms are easily distracted, and might choose to observe a large creature even while holding onto a Spear, providing the perfect opportunity to retrieve it.
Garbage Worms only retreat back into their dens after stealing a Spear due to their proximity to a creature. If somehow one gets a hold of a Spear without a creature close by, it simply holds onto it while behaving as it would without one.
Garbage Worms were first mentioned in Update #5 on the Kickstarter page.
The line "Sometimes you meet something that eats something other than you", alongside the gif just above it, might prove that the sucking behavior is actually the Garbage Worm consuming matter from the ground (which is likely waste, given the name). It's uncertain if the Garbage Worms found in-game are fully mechanical creatures, made to clean the Garbage Wastes, or partially mechanical beings which managed to survive by consuming the refuse that other creatures find inedible.
SPOILER WARNING: ??? This section contains major plot details from ???. If you have not made significant progress, completed the game as ???, or solved their riddle, then read no further!
Garbage Worms can be killed by the orange acid in Nachos Will Never Be The Same.