Slann

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

In Dungeons & Dragons, they're called "Slaad," and since they're trademarked by TSR Wizards of the Coast, you won't find them in any of the online D&D reference works. They first appeared in the Fiend Folio, aka the British Monster Manual, and described as "total devotees to Chaos," ... hmmm not suspicious at all, nope.

In 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 fled or died, and the Slann were left in charge of fixing things.

In 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 descendents.