A kind of slimy invertebrate.
Multiple specific kinds of magical slugs are mentioned: horned slugs as part of the boil cure potion in PS8, fire slugs as Newt Scamander's current research subject (FW), and flesh-eating slugs as pests in CS4 and as someone's Boggart in PA7.
Jelly Slugs are a type of wizarding sweet (PA10).
The Slug-Vomiting Charm causes the victim to belch up slugs (CS7, OP19). Ron tried to hit Malfoy with this curse after Malfoy had called Hermione a Mudblood. Unfortunately, Ron's wand had been damaged earlier, so the spell backfired (CS7).
Interestingly, Ron had only a short time before accidentally casting the Slug Vomiting Charm on himself, at breakfast, snapped "Eat slugs, Malfoy!" This is not the incantation, however, although CS/f clearly and incorrectly indicates that it is.