Plague Toads are a breed of daemon aligned to the Chaos God Nurgle, lord of decay and pestilence. They first appeared as one of the bindable monsters in Storm of Magic, though Tamurkhan: Throne of Chaos, from Forge World, would later come out with rules of its own for them. It also gave stats for Pox Riders, Nurgle's Daemonic Cavalry; Plaguebearers riding Plague Toads.
Without a doubt the most miserable and wretched of all the daemons of the Plaguefather, Plague Toads are sometimes suggested as being an unusually advanced form of nurgling, grown to absurd proportions on a diet of the most noxious filth, but are more commonly believed to be plaguebearers devolved into a monstrous, toad-like form for the crime of failing Nurgle. This theory is certainly borne out by the abuse and petty ire that the rest of Nurgle's children callously dish out to them; in Storm of Magic, it is even stated that when Nurgle is feeling upset due to setbacks in the Great Game, he likes to go into the swamps and lakes surrounding his manor and squash Plague Toads, listening to the pops, gurgles and squelches they make and watching the pretty patterns of spurting or spattered pus and filth they leave until his spirits lighten. For this reason, they are eager to leave the Realm of Chaos and enter the material world, commonly infesting sewer-pits, swamps and other places of noisome liquids and abundant foulness.
Plague Toads are also known as Sewer-Kin and Rot-Eaters.
Their canonicity is questionable; they have only ever appeared in the aforementioned books, and even in the latest Daemons of Chaos army book, Nurgle's Daemonic Cavalry was stated as being Plague Drones (Plaguebearers riding Rot Flies - the grown-up forms of Beast of Nurgle) instead. Forge World gave these guys and Pox Riders rules in 40k 8th edition, so they haven't totally forgotten about them.