Those handmade costumes are freaking sweet!
I had an idea for a pony costume myself, but a little on the simple side. I thought of getting a pair of sweat pants and sweat shirt that matched and get them whatever body color i wanted. Could always dye white ones. Then make a cutie mark and put on the side of the pants. Find some shoes and gloves to match. Get a couple of wigs for mane and tail. Acessories like horns and wings can be made or found. I spotted a woman last Halloween that was a unicorn and had the whole sweat pants/shirt thing going on with a wig and horn and a modified wig for a tail. She looked really cool.
Im tempted to do the same myself for Halloween.