I have seen one before where the person had bought a black full body blanket for the horse, (or maybe a black sheet would work) anyways they painted the sheet to match the skeleton of their horse and dressed up as a grim reaper, it looked awsome!
Sammy and I are going as Freddy Krueger and Tina Grey (first victim). He's Kreuger of course (not sure how to spell the last name. I'm going to wear white flannel PJ's and paint them with fake blood and claw marks. Sammy will have a freddy hat threaded into his bridle too. Not sure where I'm going to put the claw hand.
My mom and I made his costume. It's a blanket, the front to his hips are red and green stripes for the sweater and then he has black "Pants" draping over his butt and back legs. I have a picture of my mom fitting it to him. It's almost done. It'll have a velcro break away front.
So far he doesn't seem to care much about it. He was a bit pissed I pulled him from his dinner to fit him though.
My horse was a wild thing and I was max
Also my friend painted bones on her horse and she was the headless horseman
Another friend made the horse a bumble bee and she was a flower
Just one thing to keep in mind, if you are using paint use the crayola kids washable non-toxic paint... it comes off with soap and water, if you don't want to use paint a white soap bar will work, the soap comes off making it look like white paint//// pretty awesome