Aww, I'm sorry for your loss . Eighteen years with a horse is such a long time, and the feelings must be rather raw at the moment. No one but other horse lovers understand the depth of the relationship one has with their horse.
If you have your own property, perhaps you could plant a nice tree or something and sprinkle some of the ashes as you plant.
We have had 4 horses cremated over the years and my mum has a portion of the ashes of each . They're in the same sort of size box that you get for human ashes and she keeps them all in a sideboard with 2 lots of dog ashes and my dads ! Sorry for your loss x
So sorry to hear about your lose. I have thought a bit about this having 2oap's one especially been a very special horse and I have currently had her for 17years since she was a 4yearn old. My thought is to put her ashes up a local hill which anyone that knows my mare knows it as the bucking hill, as she loves a good canter/gallop up there after a few rather large bucks.