Slann are frog-like creatures from both of Games Workshop's flagship game lines.
Warhammer Fantasy Battle
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.
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.