Here's what the base pony looked like when I first pulled her out of the bait box:

Yep. That's...very different from where we ended up. I went with a Sweetheart Sister because a long line of artists before me have proven that the slimmer bodies and longer legs make them well-suited to deer-based customs. This particular Flower Burst was mangy, tailless, and severely discolored, with scratches to her paint on both sides. So of course the first step was getting rid of what was left of that paint and hair.

You can see here that I split the hooves first thing, and started sculpting at the feet because I knew that would be an easy bit to get out of the way. I also bored holes with a heated sewing needle and inserted guide wires for the antlers, securing them inside the head with Apoxie Sculpt.

This is where his face starts to take shape into a broader, less delicate muzzle with the beginnings of a distinct nose instead of horse nostrils, and where he gets his larger ears. This is also obviously where I sculpted the antlers. This was my first pair, so I wanted to make sure I could get them right before I got deep into the rest of the sculpting. They are probably the most show-accurate part of his design; I spent ages counting points on screencaps and making sketches to make sure I got them in the right places.
If you look closely at his back legs you can see my first attempt at sculpting the feathering down his hocks, which I scrapped shortly after this for looking too beardlike and not feathery enough.

His heavy brows, and the first few layers of his feathered ruff. You will begin to understand why making him took nine months when I tell you that each layer took half an hour to an hour to place, then had to cure for at least four hours before I could start the next one.

Starting everything at the front and working my way back was a wise choice as far as getting the look of him right, but structurally it was not wise and he was pretty much falling over if I breathed on him too heavily. So I improvised. The little fanny pack is made of ribbon and tulle left over from other customs, and the board game stones inside made a perfect temporary counterweight to keep him on his feet.

I debated giving him sculpted eyes for a good while. My original plan had been to just use the eye indents from the original pony, but adding that tiny bit of clay made a huge difference in the intensity of his expression.
(Not pictured: replacing the fanny pack with internal weight by carving a small hole in his backside, filling his legs with sand, then covering the sand with plaster of paris to keep it in place.)

The beginning of his back end. You can see where I thickened up his haunches a little just to give him some more muscle definition.

The next stage of his back end, which started with an oversized deer tail and then got elongated feathers overlaid on top of it.

The final stage of his tail sculpting, and in fact all of his sculpting. An extra line of shaggy feathering from hock to hoof, and a triangle of layered feathers to integrate his tailfeathers with the main body.
(Not shown: Sanding and gap-filling. So much sanding and gap-filling.)

The "wendigo" stage. I started off his painting with an airbrushed coat of wrought iron grey, which became the basis for all the other colors I layered over him. The other shading on him is all layers of lighter grey and true black, with a touch of blue in his antlers and some dioxazene purple in his eyes.

You'd never guess that this started out as the pretty blue pony at the top of the page, would you?