You can do a search on ebay
for bridle plume. Should pull up a picture of an upside-down T shaped device with feathers in it. The short part, top of T, has a hole in each end to fasten to the crown piece of a bridle, while the long leg of the T holds the plume.
Maybe you could make the holder yourself out of some soldered pipe or something similar. Then add the ostrich plumes. You may want to glue in the plumes so they don't work loose as horse moves head.
DO PRACTICE with horse wearing the plumes AHEAD of time, work out any issues ahead of "the Night" you will all be dressed up.
Additional decorations could include a running braided mane with ribbon, maybe like a Draft braid. Perhaps a hunter braided tail, with little flower decorations on wires twisted on both mane and tail. Some hairspray on the rump with glitter could look quite nice! Guess it depends on how decorative you want to get!!
Have a lovely evening, post a photo here on how it turned out!