Slugs are magical creatures that transform into more powerful versions of their species when they hit 100 miles per hour. Sluglings are young, newborn slugs. There are various different types of slugs in Slugterra. To obtain a slug, one can buy, trade, raise from sluglings, or capture a slug. In some competions you take a slug from your opponent if you win. The best way to train your slug is to be best friends with it. It is constantly said,"A happy slug is a healthy slug and healthy slugs win duels." There are many things slugs can do. Slug abilities range from many different elements, like fire or water. Some slugs make great strikers like the Infernus or Rammstone. Others are best at sabotaging the opponent, like the Flatulorinkus or Vinedrill. And some make the most use as a defense, such as the Frostcrawler or the Arachne. And only a few rare slugs can heal, like the Boon Doc, which can change ghouls into regular, happy slugs. There is only one slug that does not transform, and that is a Flopper.
Slugs give off a special kind of energy called Slug Energy. This energy allows life to exist in Slugterra. It allows plants and light crystals to grow. Unfortunatel, through dark science, slugs can be corrupted and are turned into monstrous beasts called "ghouls". They are only distributed by Dr. Blakk. Artificial slugs (Roboslugs) can also be built, able to hold a database of all slugs' (excluding the nearly extinct Enimgo slug) abilities and produce countermeasures.
In "Deep Water, Dark Water", it's revealed that most slugs can survive and breathe underwater without the need to surface. Slugs get to places by using underground air shafts that humans don't know of. Only slugs.
Discovered by the Shane Gang, some slugs can reach an extra powerful state called the "megamorph" when fired from a blaster with an accelerator. However, the megamorph can be acquired by only experienced slugs.