I think it stems back to the days of wild horses and wild boar - it seems a natural instinct for a horse to be frightened of pigs, although they usually become used to them over time. In East Anglia we have lots of outdoor pigs, some bridlepaths run fairly close to them and it can make for interesting riding at times
They will get used to them. We have pigs next to us and although it was a shock to the horses at first, they don't bother now. I suppose it's like anything new really.
Funny this topic has been broached because only today I brought Fred down to the farm for a bit of a pampering session and the pig (usually lives in its pen) has been let out to roam freely recently. Fred went bananas and got himself very upset!! I didn't realise it was such a common thing for horses to be afraid of pigs...
We have a fair few pigs outside in fields around - my sister's horse is not keen AT ALL! Mine couldn't give a monkeys really though - think he'd probably love to get in there with them!
It does (or is supposed to) stem back to the days of wild boar who probably would have attacekd horses. Don't know when my sisters horse or his sire/dam where ever attacked by pigs though to pass the story on!!! Guess it's like other horses and their fear of 'killer plastic bags' - must stem back to the days when lions disguised themselves with plastic.....! hehe
I have been told that pigs give out a VERY agressive 'predator' scent which the horses react too initially.
I used to keep my horse on a pig farm. He was very stressed when he first moved but by the end the farmer made me move him into a different stabling area because he was leaning over the wall and biting the piglets so they would run off and he could eat their grain!!
My old horse was a nightmare with pigs, I had to make huge diversions to avoid pig farms and there was a local show I could;t go to because it was next to a pig farm. He would snort, prance and sweat like nothing on earth and take hours to calm down!
Last year we got a piglet at the farm and my two new horses were fine with her as she grew up!
My old yard used to have pigs and to go out you had to ride past them or lead past them if out in the field and the pigs were in due to having piglets etc. Two horse fields were also in view of the pigs in the field as well. At first my lads were a bit apprehensive of them especially when they squealed etc. But some days I would catch them watching the pigs in the field. Doug would just stand there and stare at them meandering about which was amusing to see.
The horses will get used to them eventually so don't worry
My girl used to be massively scared of pigs..reacting really badly if she went anywhere near them! she was also scared of tractors but i think that was just her being slightly crazy!
Not much help tho she lived near pigs for a while and never got over the fear!