God of Evolution
This god started out pretty much normally, for a god.
He began as the usual pinch of raw ego, then gathered followers. This snowballed until he was somewhere in the middle rankings. He wasn't in the Premiership among the Offlers and Blind Ios; nor was he in the Sunday local pub league alongside Om (as tortoise), or the long-forgotten Ur-Gilash. No, he was a solidly middle-ranking Division Two god, a Stockport County of Godhood, and for a God this is a comfortable enough place to be, with a fertile valley or two of followers and nobody seeking to usurp his place.
This allowed him time to think about things, which is not a typically god-like preoccupation. And some dangerously liberal thoughts started to emerge, like What's the point about this shouting and smiting business? Getting angry all the time isn't good for the blood pressure, that is if I actually had any blood. Waiting for some poor soul to be randomly smitten by lightning and then pointing the finger and saying "See what happens to sinners? Believe in me!"
It was possibly inevitable that a God of Intellectual Inquiry had to arise on the Disc, as there is a God (or an Oh God) for just about any other quality that can be named or imagined. Therefore this God arrived, all by himself, at the same sort of notions that were forced upon Om by Brutha some millennia later. But he discovered the time wasn't right for it. He refrained from smiting out of sympathy for his believers, only to find that one of the most contrary aspects of humanity was that they promptly went off and started to worship some other, more basic, god who didn't have the same sort of liberal existential angst.
Like Om, he realised that once human priests start to intercede for you and take it upon themselves to interpret your actions to the believers, they inevitably begin to follow an agenda of their own that inevitably involves bigger and better temples, bigger tithes than that derisory ten percent the God must have ordained on a bad day, or which is clearly a misinterpretation of Holy Scripture, and let's have your very best stock animals lined up here for sacrifice, please, anything less is an insult to the Divinity...
In desperation he tried liberal commandments emphasising justice, decency, brotherhood and fraternity, only for the tribe from the valley next door, worshippers of an old-time God who had allegedly told his folk to smite and lay waste the unbelievers next door, to swoop down like a wolf from the fold and over-run his valley.
And here, as with Ur-Gilash, the story should have ended.
However, the God of Intellectual Inquiry discovered a strange and new thing. Having worked out that "everything, in the end, is just a set of instructions", he re-instructed himself, kept his God-like powers and sanity, and turned his back on humanity, the better to pursue scientific enquiry.
This is proof that even Gods can evolve - to the point where they don't really need any believers and can maintain ego-integrity and God-like abilities without them. ("Charcoal" Abraxas didn't get this far in his logical inquiries, or it would have made a great sequel to On Religion). Although it is true that in acquiring at least one believer - Ponder Stibbons - his strength grows appreciably.
Turning his back completely on the human race and moving to Mono Island, he then threw his energies into re-designing living creatures and seeking to improve on them. This is hampered as he can only do things one at a time: until he has a frank chat with Mrs Whitlow, he is ignorant in the fine details of how to get his creations to reproduce themselves. Many of his early creations proved non-viable, due to his misguided attempt to put wheels on everything; of the ones that made it, the hermit elephant can be considered typical of his "improvements".
By then, most of the wizards have taken advantage of Argo Nauticae Uniquo to leave the island. Ponder Stibbons stays - for just long enough to grasp the awful truth of how far this God has turned his back on the human race.
The future has six legs and is cockroach-shaped. Insects are the pinnacle of life. Larger animals are there mainly to provide dung, or hosts for parasites.
The God of Evolution makes a happy return in The Science of Discworld III: Darwin's Watch, in which the Auditors persuade him that Charles Darwin would benefit from his useful advice and expertise, resulting in the total disruption of Roundworld history and the publication of the Theology of Species. He is seen in his mountain having moved onto some new creatures, such as wind-propelled, wheeled elephants, dirigible whales (with inflatable skeletons), and a sea-air ballistic shark (which was, perhaps fortunately, scrapped, for some reason).