Slann
Slann are frog-like creatures from both of Games Workshop's flagship game lines.
Warhammer Fantasy Battle
"K:"The mystery of the Chaos Wastes... Who do you think created them?"
M:"Frogs. I heard it was frogs. Really big frogs.""
"FROGS? FROGS!? I'VE NEVER HEARD ANYTHING MORE RIDICULOUS IN MY LIFE!"
In Warhammer Fantasy Battle, the Slann are the lieutenants of the Old Ones, created to lead the Lizardmen, and given goofy names like Cupacocoa, Mylkbotyl, Pinacolada, and Kroak (the last one is still canon). When the Warhammer world's polar gates broke down and Chaos streamed into the world, the Old Ones either fled or died, and the Slann were left in charge of fixing things. Despite their being so fucking magic that their mummified remains shoot magic at people, they have not achieved as much fixing as one might hope. In fact, basically no fixing. Quite the opposite of fixing, actually. Thanks to their continental fine-tuning, the Dwarf empire was shattered, paving the way for Skaven and greenskins to once again become major powers in the world. But then again a fixed up utopia ruled over by giant frog dudes on palanquins wouldn't be a great setting for a wargame. Maybe an OK one for FATAL, because those guys are into toad love, but not for slamming minis together and rolling dice so much that you can't jack it for a week.
It also didn't help that at some point, Slann just...stopped being made. The spawning pools all lizardmen rose from just kept making more of the other scalyfolk, making each Slann irreplaceable.
Oldhammer
Whilst the Slann are remembered as the bloated toad mage-kings of the Lizardmen, that's actually a relatively modern development. Both races go all the way back to 1st Edition, but they were initially an entirely separate race!
Back in 1e, Slann debuted in the scenario "Legend of Kremlo", before making it into the Forces of Fantasy splatbook for 1e. Here, they were portrayed as essentially a race of frogmen with an Aztec-themed society; they had once controlled a vast empire that linked mighty city-states built around huge ziggurats with great highways, but in the present their empire had been hugely reduced by incursions from Norscans and elves; only the core of the Slann empire remained intact, and many slann had devolved into primitive tribes living a hunter-gatherer lifestyle in the tropical rainforests surrounding the remaining slann city-states. They could field wizards, but those wizards were nothing special, and they also fielded many slann warriors, armed with blowpipes, daggers, swords, tomahawks and exotic polearms called "water scythes", with their ranks further bolstered by drug-addled castrated human slave-soldiers.
In 2nd edition, Slann society appears in the corebook, which builds upon the minor lore from Forces of Fantasy. Here, the slann are said to have once had a mighty technologically advanced society, one that made them "lords of time and space", as the book puts it. However, in the modern era, they are nearing the end of a long, long fall from grace, having descended into barbarism and turning their backs on the technologies that once empowered them. Their grip over Lustria is waning in the face of human encroachment, and the more primitive tribals are even beginning to abandon what is left of their culture in favor of aping humans - presumably, there is still a lingering slann empire at the core of the territories controlled by the tribal primitives, but it's not explicitly mentioned. Imperial slann are the only forces capable of appearing in the game, where they act as leaders to lobotomized, castrasted, drug-addled human slave-troops. Again, they have no special relationship to magic, although they are capable of becoming wizards. Lizardmen, in contrast, are restricted to the Ravening Hordes splatbook.
In 3rd edition, the future of the slann began to take shape. Here, they appear in the corebook and in Warhammer Armies as one of the defacto army lists, and for the first time, they are explicitly associated with the lizardmen - in 1st edition, their only connection was that they would ride Cold Ones, believed to be a relative or degenerate form of lizardman, and in 2e that lore was lost. Simply put, 3rd edition represents the single greatest body of lore on Oldhammer slann!
Their lore is based on their 2e lore, with the bulk of it to be found in the corebook. New additions to the lore have the Slann in the role that would later be taken over by the Old Ones in 5th edition; an advanced alien race that descended to a primeval world and decided to colonize it, reshaping its geography and slaughtering the native races that attempted to defy them or which they simply viewed as unnecessary. Of these, one of the few survivors are the lizardmen, now reduced to scattered barbarian tribes - ironically, this lets the slann field lizardfolk and troglodytes in their armies, representing vassals forced into service. They now have an Emperor, named Mazdamundi... yep, this is where Lord Mazdamundi began! A new addition to the lore is the existence of warrior brotherhoods based on totemic beasts amongst the "high" slanns, as well as the primitive tribal warriors rounded up from their jungle homes. They still field the lobotomized eunuch slave-warriors of past editions, but these are described as a recent addition to their forces, made up of captive would-be human settlers to their lands.They can also field pygmy troops as allies, and are known to be allies to coatls.
Slann wizards also finally get some lore; they can freely wield Battle Magic, Daemonic Magic, Elemental Magic, Illusionist Magic and Necromantic Magic. Also, slann wizards believe their powers depend on their spiritual purity, and that touching the earth is a defiling act; as a consequence, they are born into battle atop steeds or litters (usually carried by human slave-things), and should they be forced to walk, then no matter how powerful the slann wizard, they can't cast any spells higher than 2nd level! This edition also establishes slann mages as the most powerful and learned of all the races in the Warhammer world, still hoarding vast treasure troves of arcane knowledge that human or elf mages would kill to get their hands on.
In 4th edition, the slann and the lizardmen both vanish complete, with no mention of them to be seen. It wouldn't be until 5th edition that we would see them return in the first "Warhamemr Armies: Lizardmen" army book, in which they abandoned their old lore and became the slann as Warhammer knows them today, their original lore only surviving amongst those who preserve Oldhammer lore. Like us.
Age of Sigmar
With The End Times resulting in Lustria being fucked beyond all comprehension by Skaven blowing up Morrslieb and the mightiest Slann dying, those that remained just said "Fuck it!", turned their pyramids into spaceships and ran off into space before Archaon blew up the world. Eventually they bumped into Dracothion in the void and told him what happened to the old world, which caused the dragon to run away crying. The Slann followed his tears to the realm of Azyr, and decided this was as good a place as any to settle down.
Unfortunately, the Slann were the only folks who could survive the voyage across realities, as the lizardmen attendants all eventually withered and died... until GW decided to retcon that, so now they have spawning pools inside their ships and have actually been mortal creatures this entire time.
Warhammer 40,000
In early editions of Warhammer 40,000, the Slann were the oldest space civilization, even older than the Eldar; in fact, the Eldar learned from them when making the Webway. For reasons never really elaborated on, they mostly died out by the 41st millennium.
When Games Workshop made their game more grimdark in later editions, the Slann were effectively replaced by Old Ones, who mostly died out in the aftermath of the war with the Necrons, though some still survived to advise the Eldar and warn them about the coming Fall. The third edition Necron Codex does contain a nod to their origins: it suggests that Lizardmen miniatures could be converted to stand for the Old Ones' distant descendants.
However, they don't really appear much other than being mentioned as "having been created by the Old Ones", followed by "helped the Eldar", and then apparently fucking around doing nothing for millennia, and then dying out. Presumably the translation of large floppy blubbery creatures into zero gravity made them too mentally distressing to want to talk about in the fluff too much.
What is implied to be a Slann makes a cameo in the book "Old Earth". It's never stated to be a Slann, but it certainly fits the bill as a giant frog creature that lives in a mesoamerican pyramid. It was one of the top leaders of the Cabal until Eldrad killed it.
The Votann are kinda like Slann, but in computer form. Both are "ancient" creations that are the guiding minds of their culture, but are hampered by their inactivity, taking decades to even make a simple decision.