Official D&D settings have, pretty consistently since first edition, the same set of monstrous and demihuman deities. They even have some of the same evil deities.
Flipping through the Monster Manual shows a lot of lore around the various monstrous or demihuman deities mentioned. . .Grummsh, Ilsenine, Corellon Larethian, Garl Glittergold, Kurtulmak, as deities presumed in the core rules of most editions.
Tiamat (and Io and Bahamut) are present in almost all of the official D&D settings. . .even in Eberron, which normally doesn't have the same Gods as the rest of D&D (albeit their role in Eberron is very minor, they are canonically present). Tiamat was even such an iconic villain of D&D that she was the big villain of the 1980's D&D cartoon series.
Evil deities such as Tharizdun or Asmodeus stretch across multiple editions, often mentioned right in the core rules themselves, and when not in the core rules, they're often depicted in generic works not tied to a specific setting.
From reading D&D core books and supplements, not tied to specific settings like Greyhawk, Forgotten Realms, or Eberron, you'd certainly conclude that there's a certain group of deities that are presumed to be in D&D games (unless the DM says otherwise).
. . .but all those deities are either patrons of demihumans or monstrous beings, or are evil in nature. There is a conspicuous lack of a generic good setting-neutral counterpart to Asmodeus as the archetypical lawful evil deity, or Tharizdun as the generic chaotic evil deity. There's never been a human racial deity or pantheon the way the Seldarine are to Elves or Moradin is to Dwarves, or Tiamat and Bahamut are to Dragons.
As I was writing this, I thought of a counter-argument, 3e DID use an abbreviated version of the Greyhawk pantheon as a generic pantheon in the core rules (and adding deities to the core rules was a change I welcomed), but this wasn't followed in later editions, so it really isn't a constant across editions the way that the above deities were. 4e had it's own list of PHB deities which was largely, but not entirely, drawn from Greyhawk (plus Bane from the Forgotten Realms, for whatever reason). The 5e PHB included a short list of the major deities of the official worlds and historic pantheons (including such breadth of deities in the core rules was one thing 5e definitely did get right, in my opinion).
If you included deities that were mentioned in at the PHB's of 3 editions, you'd have Pelor (as the only good-aligned deity that generally accepts human worshippers), Kord (the similarly neutral deity for Humans), and Vecna (yet another evil deity, but as Vecna wasn't generally seen as deific before 3e, I didn't include him with deities that had been in core or setting-neutral D&D lore since AD&D days). If you included deities that were mentioned in the PHB's of 2 editions, you'd have the group of core Greyhawk gods from the 3rd edition PHB, which were also mentioned in the 5e PHB, plus Bane from the Forgotten Realms.
Anyone else ever think about the presumptions of core deities in D&D?
