I don't know if it's just me but doesn't the Skeleton King look very similar to Skeletron from the hit video game Terraria https://terraria.gamepedia.com/Skeletron I would probably know because Terraria is my favorite game
Here are my reasons
1. Skeletron is slit into 3 parts, 1 head, and 2 hands. So is the Skeleton king not to mention they're both skeletons.
2. The Skeleton King's Lair looks oddly simmiler to the dungeon from Terraria, and well its Pixel dungeon, they had to have seen the simulatities.
3. Both the Skeletron and the Skeleton king inflict debuffs, the Skeletron inflicts bleeding and slowness, where as the Skeleton King inflicts weakness and slowness.
4. The Skeleton King drops and adamantite weapon, which does not have much simularites to Skeletron itself, but there is an metal called adamantite within Terraria, so maybe they got the adamantite idea from Terraria