These evil froggies are native to the Greyhawk setting (which probably accounts for their long absence from the public eye), and first made their appearance in 1988 in the splatbook "Greyhawk Adventures" for Advanced Dungeons & Dragons 2nd edition. They were subsequently reprinted for the Monstrous Compendium Appendix Greyhawk, also known as MC5.
Following this, they basically up and vanished for... like, forever, before surprising everybody by reappearing in Volo's Guide to Monsters in Dungeons & Dragons 5th Edition. The reason why? The whole 5e team each got to put one monster they really liked in that book, and one guy was a Greyhawk fan who somehow remembered that grungs existed. And so here we have them... ironically, 5e grungs (perhaps better called Forgotten Realms grungs) are actually pretty damn different to the original grungs, and are much more like evil, poisonous counterparts to the grippli. Whether or not this is a step up from their original selves, who were basically bitey, poisonous bullywugs, is up to you.
Greyhawk Grung are highly territorial, 3ft tall humanoid toads or bullfrogs who inhabit swamps and marshes... no duh, were you expecting them to live in deserts? Whilst they are an intelligent, bipedal, tool-using race, evolution has robbed them of some of their more bestial abilities; grung are incapable of making the prodigious leaps normally associated with frogs, and likewise have lost their ancestral long, prehensile tongues - in stark contrast to their bullywug counterparts. However, they have gained mouthfuls of razor-sharp fangs, so they're not completely screwed over. That said, they still breathe through their skin, and so they need to keep their slick hides nice and moist. Grung are colored like bullfrogs; dappled green & brown backs, contrasting white or yellow bellies. Their eyes — smaller than those of frogs and toads and protected by bony ridges — are red with black pupils. Males are slightly smaller than females.
One bestial ability that they do have, which bullywugs and grippli lack, is a skin that secretes toxic mucus. By rubbing a bladed weapon on their bodies, it becomes coated in venomous slime, which remains volatile for 10 rounds; victims cut or pieced by such weapons must succeed on a Save vs. Poison or die in four rounds. The same slime also impregnates their saliva, so a grung bite is also deadly; it's slightly diluted, but not enough to save you (+2 to the saving throw, death in six rounds). Add in that grung have long realized they can compensate for their small size through ambush tactics, and... yeah, they are nasty little buggers.
Not helping is that they are stupidly aggressive. Grung villages claim all territory within at least a mile of their tribal settlement as theirs, and will attack any outsider from their tribe that enters this area - when not ambushing members of other races, grung fight constant border clashes amongst their own tribal units. Such is their militant zeal that they will never negotiate; fights are to the death. This may be partially because grung are cannibals, and consume literally anything can catch, including humanoids and grung from other villages.
For obvious reasons, most swamp-dwelling cultures want to get rid of grung as soon as they can. Especially because the presence of a grung village poisons the water around it due to the many toxic toadlings swimming in it; any non-grung drinking the water must roll a successful saving throw vs. poison (with a +3 bonus) or become nauseated for 2d4 rounds. Nauseated creatures fight with penalties of -1 to their attack rolls and +1 to their Armor Class. That said, their poisonous skin protects them from the predatory attentions of most creatures, except those that are themselves immune to venom - such as certain species of giant snake. Grung poison is almost impossible to bottle (as any exposure to air causes it to decompose in ten rounds), so your players shouldn't get any bright ideas about trying to use that nasty sludge for their own uses.
A grung settlement is an untidy collection of crude shelters, sometimes concealed inside large dead trees, with ready access to open water; both for the grung to moisten themselves in, and to house grung eggs, as grung go through the same egg-tadpole-toadlet-toad lifestyle of any frog. Grung tadpoles become grunglets after three months and fully adult grung at the age of nine months. A tribe typically consists of about 10d6+40 adult grung, and another 25% of that number in grunglets.
Weirdly for a savage humanoid race, grung tribes are matriarchal, perhaps because the females are bigger and thusly stronger. War chiefs are all female, and the tribal chieftain is the strongest fighter among the war chiefs. Rising through the ranks is by duels to the death, with the victor getting the title. Each tribe also has a single female shaman of up to 3rd level. Her spheres are All, Animal, Combat, Healing (reversed spells only), and Plant.
According to the Oerth Journal #8, Wastri is the official patron god of the grung, which makes way too much sense when you think about it.
Forgotten Realms grung are brightly colored, aggressive and territorial humanoid tree-frogs native to the tropical rain forests and jungles - they are only found in areas with plentiful moisture, since they require pools to mature their tadpoles in, and must spend at least an hour a day bathing, or else they'll begin to fatally dry out. Arrogant and elitist, these Lawful Evil creatures divide themselves into a strict hierarchy, which places grung above other races and divides the grung into different castes based on their color. This might be because whilst all grung secrete a poisonous slime - which can be used to enthrall humanoid creatures, thus allowing them to assemble great legions of slaves - each color of grung is able to induce a unique flavor of madness when they afflict a creature with their slime in its natural form, as opposed to the diluted "thrall-juice" they normally use.
Green grungs are the tribe's warriors, hunters, and laborers, and blue grungs work as artisans and in other domestic roles. Supervising and guiding both groups are the purple grungs, which serve as administrators and commanders. Green grung slime induces an uncontrollable urge to either climb the nearest high place, or hop around like a giant frog. Blue grung slime compels the afflicted to shout, scream, or otherwise make the loudest noise that they can. Purple grung slime induces a maddening sensation of being boiled in one's own skin, compelling the victim to find cool water or mud to soak themselves in to alleviate the psychological heat.
Red grungs are the tribe's scholars and magic users. They are superior to purple, blue, and green grungs and given proper respect even by grungs of higher status. The slime of these grungs induces an insatiable hunger.
Higher castes include orange grungs, which are elite warriors that have authority over all lesser grungs, and gold grungs, which hold the highest leadership positions. A tribe's sovereign is always a gold grung. The slime of an orange grung overwhelms a victim with paranoid delusions, whilst golden grung slime enthralls the victim, compelling them to obey the golden grung's orders and even magically enables them to understand the grung's language.
These tree frogfolk inhabit shaded treetop villages, centered around assorted well-guarded ground-level pools, where their eggs must incubate. Like Greyhawk grung, it takes nine months for a grung egg to reach full maturity, spending three months as a tadpole. Each caste maintains its own hatching pool, and the juvenile grunglets - which are all a dull greenish gray until they reach maturity - join their birth caste immediately after leaving their pools. That said, grung are meritocratic, to a degree; on rare occasions, an individual that distinguishes itself an earn an invitation into a higher caste - a process centered around ritual spells combined with herbal tonics. These lucky grung permanently ascend to their new status, and their offspring will likewise belong to that caste.
Whether this means that grung can be punished by being forcibly transferred to a lower caste isn't brought up in the book.
As stated above, grung are slavers, forever on the lookout for creatures they can capture and enslave. Grungs use slaves for all manner of menial tasks, but mostly they just like bossing them around. Slaves are fed food that has been spiked with a carefully diluted form of grung slime, just powerful enough to keep them lethargic and compliant without afflicting them with madness. A creature enthralled in this way over a long period of time becomes a shell of its former self and can be restored to normalcy only by magic.
Somebody at Wizards of the Coast had a wild idea to run a game of Tomb of Annihilation where all of the PCs were grungs, and wrote a supplement for Tomb of Annihilation called One Grung Above which gave PC stats to grungs.
Ability Score Increase: Your Dexterity score increases by 2 and your Constitution score increases by 1.
Age: Grungs mature to adulthood in a single year, but have been known to live up to 50 years.
Alignment: Most grungs are lawful, having been raised in a strict caste system. They tend toward evil as well, coming from a culture where social advancement occurs rarely, and most often because another member of your army has died and there is no one else of that caste to fill the vacancy.
Arboreal Alertness: You have proficiency in the Perception skill.
Size: Grungs stand between 2 ½ and 3 ½ feet tall and average about 30 pounds. Your size is Small.
Speed: Your base walking speed is 25 feet, and you have a climbing speed of 25 feet.
Amphibious: You can breathe air and water.
Poison Immunity: You’re immune to poison damage and the poisoned condition.
Poisonous Skin: Any creature that grapples you or otherwise comes into direct contact with your skin must succeed on a DC 12 Constitution saving throw or become poisoned for 1 minute. A poisoned creature no longer in direct contact with you can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. You can also apply this poison to any piercing weapon as part of an attack with that weapon, though when you hit the poison reacts differently. The target must succeed on a DC 12 Constitution saving throw or take 2d4 poison damage.
Standing Leap: Your long jump is up to 25 feet and your high jump is up to 15 feet, with or without a running start.
Water Dependency: If you fail to immerse yourself in water for at least 1 hour during a day, you suffer one level of exhaustion at the end of that day. You can only recover from this exhaustion through magic or by immersing yourself in water for at least 1 hour.
Languages: You can speak, read, and write Grung.
Disclaimer: Outside of a certain surrogate character, this document in no way makes grung a legal, playable race in the D&D Adventurers League, or any other campaign where the Dungeon Master hates amphibians with an excess of apostrophes in their names… and yes if you’re not immune to poison and you must touch the grung to heal it… you get to make a saving throw.