Question: So, as I understand it, wheels are almost completely absent in nature, in terms of a method of mobility. Would it be possible, or reasonable for such a creature to be made along the lines of evolution? If so, how? Could such a creature achieve such size as to be big or fast enough to be a mount?

I hypothesized that an insect like creature with feet that formed a circular set of smaller legs could then use them as spokes, moving the outer rim of a wheel in a circular motion. Where they'd get the wheel is anyones guess, but it was a start, I suppose.

Answer: 
If you want to have a wheel-like subsystem, you need to solve a major problem: how to isolate the rotation of the sub-system from the remaining elements.

On a car or a chariot this is achieved by bearings, but this is not feasible in living organisms, where the need of having "cabling", normally called blood circulation system and nervous system, going through every part of the body necessarily limits the degree of relative motion between adjacent parts.

This doesn't mean evolution didn't discover the wheel. We have [examples](https://www.lucianabartolini.net/pagina_ordine_isopodi.htm) of [animals](https://en.wikipedia.org/wiki/Armadillidium_vulgare) which use the sphere as configuration, partially also to enable quick escape. But that is used at system level (the entire body), not a sub-system (one or more limbs).

[![enter image description here](https://i.stack.imgur.com/lY4NX.jpg)](https://i.stack.imgur.com/lY4NX.jpg)

[![enter image description here](https://i.stack.imgur.com/e4Fub.jpg)](https://i.stack.imgur.com/e4Fub.jpg)
In extremely complicated theory its ofcourse possible. In practice not. Evolution happens because a genetic trait gives a teeny tiny advantage which causes more with that mutation to survive and eventually spread the mutation to everyone.

The golden wheeled spider can form a sort of Ball with his body and limbs and then roll off of sand-dunes to safety. (near the end of the video)
<https://youtu.be/ltvLo2vNls0>

The thing is, his rolling capacity wasnt an advantage until he could well and truly roll. Any creature that has wheels for limbs needed to go through an evolutionary period where his wheels arent complete and need to offer an advantage without being able to roll. It has happened a lot that first one trait of an evolution was handy, and that this managed to develop most of another evolution that wouldnt have worked otherwise. But what evolution would favor the dangling half-wheels to form far enough to go to full wheels?

Theres also the complicated matter of getting blood and nerve-connections into the spinning wheel. Its fully possible to create something like that, but not easy and probably impossible for evolution to create. You also need a bunch of mini-limbs that push the wheel to roll, or break the wheel when the creature needs to walk.