You should NEVER feed horses that aren't yours. NEVER. You shouldn't feed ANY animals that aren't yours unless you have the express permission of the owner. It's in the country code, which I would hope all Geocachers follow when out and about!
Most of these stories of bolshy horses would be very well explained by a history of people walking through those same fields and giving the horses something to eat. Even just a handful of grass.
Why does feeding horses make them bolshy?
Because if people who walk through the field feed the horses, then after a few times the horses will expect to be fed. When people walk through the field and don't feed them, the horses get upset because they thought they were going to get food. After a few months of getting fed by the odd person who walks through the field, most horses will start getting a overly friendly with the people who don't feed them. After a year they will start getting pushy and after a few years they can get very bolshy, threatening, and yes some will actually bite if too many people have gone through the field without feeding them. This is absolutely NOT a reason to feed horses who are bolshy with you - it will only make the situation worse.
My mum has had ponies for as long as I can remember. I've grown up around them, as did a number of my childhood friends. I've seen several of their ponies turning like this, simply through being kept in a field to which the public has access. I've watched my mum's ponies turning like this because the only field she can rent has a footpath in it. One of the ponies we've had since I was five and he was three. We've grown up together and I knew him very well. I moved away a few years ago, just after the ponies moved into this field. I don't know him well at all now, even though I go home often. He's a very different character. Because we know walkers through the field feed the ponies - we've found the evidence.
I might add also that feeding horses you don't know can make them very very ill. There's an illness called Laminitus, which is caused by the horses eating too much rich food. They can get it on their own very easily if the owner just chucks them out into a field, because during spring and autumn, when the grass is richest, the ponies will eat too much and fall ill. The horses can get Laminitus even easier if people feed them outside of what the owner knows they are getting, because then the owner cannot adjust for it.
Laminitus causes a bone (the pedal bone) in their foot to rotate and move downwards. This is very painful in itself, but it's not all - in moving down the bone builds up a huge pressure on the bottom of the horse's foot, which means that they find it very very painful to stand up. However Laminitus also causes horrendous stomach ache, so it is very very painful for them to lie down as well. Not only that, but since horses cannot throw up, the stomach ache won't go away, and they can be suffering for days, weeks sometimes.
Once a horse has had Laminitus once, they can get it again even easier. The pedal bone may not return to it's original place, and may continue to cause the horse aches or pains.
Finally, Laminitus can be fatal.
Other reasons for bolshy horses, is some dog-walkers letting their dogs run around and worry the horses - the dog doesn't even need to be in the same enclosure as the horses for it to worry them. A few months of being worried by dogs, and horses will start getting very defensive. Some of the horses of a less mild temperament will be a little more assertive in their defensiveness.
As for what to do when encountering bolshy horses, well it's different with every horse, as many people have said. I'd say the best thing to do is, as everyone else has said, talk calmly and quietly to it, don't necessarily try to stroke it (agree wholeheartedly that a head-butting match with a horse will result in a collection of bruises), and exit the field calmly and without fuss. Running around panicking will cause the horse to run around panicking. If you feel nervous, don't talk to the horse - they can hear your nervousness and it will make them nervous. I've never once met a horse who will hurt someone when free in a field, and I've met a lot of horrible little pony club ponies, including some I've looked after. That's not to say they don't exist - I wouldn't know. But the chances of you meeting one are very very small. So you need to just keep calmly, quietly, moving towards the exit of the field.
