How to get a horse used to Dewormer
The best way I found to get a horse to accept dewormer, is to for a few weeks mix molasses and a bit of water into an empty syringe the size of a packaged product dewormer.
Do this daily and pick a day to use the real syringe, smear a small amount on the tip and deworm!
If your horse doesn't like molasses you can use something the horse likes IE: applesauce, etc