Swear Toads are mentioned a few times throughout in the series, a minor running gag. What makes them different from normal toads is that fact that they spontaneously shout profanity, presumably in Gnommish. They are also mildly venomous. According to The Artemis Fowl Files, Holly Short was once bitten in the behind by a swear toad, causing much swelling. She quotes that she hopes Artemis Fowl will never find out about that.
They started as a practical joke, but became a nuisance, which, according to Holly, "Someone lost their wand over that one." Apparently, a few college warlocks were messing about and gave toads the power of speech. Being college students, however, they only gave it the power to speak in profanity. An unforeseen side effect of the magic was fertility, which allowed the toads to quickly reproduce and spread, infesting every damp patch in Haven.