One of the strangest Dungeons and Dragons settings, and also among the more obscure, is the Known World/Mystara/Hollow World setting.
(If you're curious and don't mind reading a rather lengthy primer, this thread at RPG.net covers the setting with a focus on worldbuilding and identifying both the original starting point and the numerous, numerous retcons. Of interest to me is that the original basis for the setting, before it officially became part of D&D, was the homebrew pre-D&D campaign created by a couple of gamers in Ohio which previously included Kzinti and Tharks among the more standard fantasy races. Bit neutral on the Kzinti because they've become the model for basically every cat alien and fantasy race since, but an RPG where you get to play as a green Barsoomian? Yes please. ...Although they intended the Tharks as NPCs. Work in the Kaldanes, preferably also playable, and we've got something here.)
Part of the reason it's so strange is because it had a lot of continuity creep and was never the most popular setting, and it also got some very context-light video and computer games set in it.
Case in point: The following sequence actually makes a pretty fair amount of sense in the context of the setting. In a random game that doesn't include more than minimal worldbuilding, though...?
It results in the wonderful YouTube user comment "I love the fact that the ultimate objective is to summon Godzilla to come kick the [ahem] out of an eldritch abomination that would be right at home in a Godzilla movie." Somebody in the thread linked above commented that a non-D&D friend had been under the impression that the game had taken place inside a giant alien spaceship.
If you're wondering why a dinosaur showed up to beat up a tentacled thing, long story short is that a dinosaur named Ka became a godlike being called an Immortal a really long time ago. Being a pop-culture dinosaur, he was saddened by his kin's extinction or something, I guess, so he used the hollow interior of a planet to preserve societies and species that would otherwise have been wiped out using an elaborate system of magic and an artificial sun.
But there were eldritch abomination things called Burrowers that occasionally popped up and caused problems, if I recall correctly because they'd originally been used to dig and seal the holes that were used to access the interior at first.
So telling Ka about it prompts him to come and help, because he's the one who started the whole thing and it's kind of his project.
Yes, a dinosaur is responsible for creating what amounts to a planetary ark. It's kinda awesome.