Cap'n Kobold
Hero
Since you're mentioning Eberron, it might be worth asking what planar system you use with it.No other type of creatures in the game have been so ossified. Giants in Greyhawk and Giants in Xendrick are quite different. Genies have a bunch of different approaches. Humanoids have so many variants that you could print an entire Monster Manual with nothing but variations on baseline humanoids and probably still have enough material for a second book.
But demons and devils? Nope. They all come from the same places that they came from when Greenwood wrote those articles about the Abyss and Hell way back in the double digits of Dragon magazine and we cannot have any variations whatsoever.
Eberron doesn't have the Abyss and the 9 hells. Shavarath hosts both Demons and devils, and both Fernia and Shavarath host pit fiends as examples.