My Little Pony
This could be an interesting costume - dye his mane and tail purple and put some stars, hearts whatever on his ass. I actually do not know what any of those ponies are called. Perhaps he could be a Halloween style little pony with a candy corn cutie marks ( I was informed by a 9 year old that those ass stamps are cutie marks). However, I have waited too long. The weather has cooled and Casper is in a full winter coat that is now covered in green poop stains. Unless I can devise some kind a dirty pony equivalent to the Peanuts Pig Pen, I don't think this will really work.
The Pale Horseman of the Apocalypse
Now I think Mr. Bones would be up for playing this part but how to attached him to the saddle so he would stay put and not fall off is another matter. Casper would just have to be himself for this role.
Such a no brainer - cut a couple of holes for his eyes in a white sheet and drape it over him. I am sure he would wear it for a few pictures anyhow. After all, he is a ghost paint. Plus, now that the weather is cooling and he has a full winter coat, there is no chance for a bath, making this option awfully appealing.
I think a small cardboard sign around his neck with "will work for food" would just about sum up how Casper feels about life. Plus, his green poop stains, and the smell of stale urine he will carry with him during the winter would fit this costume nicely.