I would just stick the horse in with a pig.
I've kept pigs. They may look cute (to some...), but they are scared of nothing, they are brutally strong, they can move fast, and they will eat ANYTHING. Put your horse in with pigs if you want it dead.
- sorry, I know Foxhunter has already addressed this but I had to reiterate because this is important to know -
Apart from that, I can add that in Shropshire where I used to live there were a lot of pigs kept on the farms we rode past. We all 'got our horses used to them' sufficiently to ride past the pig pens without skittering across the road, and we did that through repeat repeat repeat of taking the horse past the smell, under control or in company or whatever was appropriate.
My mare was always snorty and aware when riding past pigs, but manageable. This was the stage most of the horses got to. Only one friend had a horse completely chilled about pigs and she had been kept in a field with pigs in the neighbouring field.
I once heard a lady on the radio tell about being chased by the wild boar in the New Forest while riding out with her horse. She and her horse were well used to riding in the forest, and the horse was accustomed to the smell of the boar. But on that day she had accidentally ridden into a copse and got between the mother boar and her litter. Whether her horse or her were the most scared I don't know but both woman and beast knew that they were fleeing for their lives.