im no expert on the origin of braiding,but if i had a guess and what i always thought was.that the feild/hunt horses,needed their manes braided to keep their manes from getting caught on things when going through countryside and jumping fences..lol hahah i don't know..and probably england is another idea....have you tried doing a google search about it to see what it came up with about it?
in this century obviously its a turnout plus.etc
hopefully someone else will reply with somemore or better insight on this topic.
I read somewhere... and I've read too much to remember exactly where...
"Blood" horses (or thoroughbreds) had finer, thinner hair than the cold blooded or cob horses more native to England. This was because they had a lot of Arabian/Barb blood, and the desert breeds had finer coats and manes because of their environment.
So, naturally, you would want to preserve this fine hair as an indication of "blood" so everyone could see the good breeding of your horse. But you needed to get it out of the way, because no one wants to hunt a horse when you're getting your fingers and reins all tangled up in that stuff. So you braided it up real fancy. If your hunter was just a cold blooded hoss, you'd roach his mane, and dock his tail.
The thinning or braiding of the tail was to prevent twigs and brambles from getting caught up along the dock with alarming consequences (bucking and kicking).
As someone who has hunted, I can tell you that keeping hands on reins that are wet with sweat--necessary for a puller-- his hard enough without mane in the mix.
I also suspect that keeping the mane out of the way was an even bigger deal when people hunted in double bridles and also carried a proper whip with the long lash. Just a lot to keep straight on the right.
I have also been told, as Smart Alex already mentioned, that it was a way to differentiate your blood horse from the halfbreds and farm horses. All the horses had something done with their manes to keep the excess hair out of the way of double bridles and whips (and flasks? ), but the common horse were roached, while the blood horses were braided.
I always was told braiding was functional in the hunt field. There was a number....seven for geldings, eight for mares - something like that. Anyway, traditionally everyone carried a hunt whip so taming the mane did prevent one's whip from becoming tangled.
Military horses generally didn't get braided. They got roached. (To this day they take the whole mane and forelock off at the Old Guard Caisson platoon to maintain a uniform look on the funeral horses.) They didn't have time to waste fussing with braids. (And given the number of funerals they do, they still don't. When you have to bathe and groom and harness six drafts/draft crosses per team with two/three/four teams working per day and have to clean all your own tack--the guys apparently never want to be sitting on the wheel horse, who has the most straps, at the end of the day--roaching is much more user-friendly.)
Even number for a mare. Odd number for a gelding or stallion.
I always find that stat hilarious - what, you couldn't tell the difference between a mare and a stallion otherwise?
Although fair enough, often stallions' forelocks are left unbraided.
An odd # of braids on the neck helps avoid the "neck broken in half" look that can happen when you have an even #.
As to the history - no idea; maybe another throwback to the Victorian era, when they couldn't abide to see anything left plain and simple and natural? Think of women's hair back then - not a natural head to be seen!