"Eat slugs, Malfoy."
--Ron Weasley (CS6)
A slug is 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, the CS/f clearly and incorrectly indicates that 'Eat slugs, Malfoy!' is the charm for the Slug Vomiting Charm. In the text Ron tells Malfoy to 'eat slugs' in CS6, but doesn't attempt to cast anything on Malfoy until CS7, (which is several days later), after Malfoy calls Hermione a Mudblood; and then only exclaiming, "You'll pay for that one, Malfoy!" (CS7). A loud bang and a flash of green light coming out the wrong end of Ron's wand is what results.
The word 'slug' appears 40 times in the series. Some appearances are when Harry feels like the Dursley's treat him like a slug (PS2), when the little wizard, Kevin, uses his father's wand to inflate a slug at the Quidditch World Cup (GF7), and in the title for Professor Slughorn's 'Slug Club' (HBP).