Mane hair cinches have been around for a very long time. The earliest ones I have seen were made in the 1870s, but they have been in use long before then.
Most of the surviving cinches are now in museums and the ones usually in the best condition are those used by the U.S. Army cavalry on McClellan saddles. These are usually black and very short. I've seen a number of these in museums that look as though they've never been on a horse. That is a real possibility as they may very well be Army surplus.
Most mane hair cinches in use today and most of what I make are purely for decoration. However, all of my cinches, even the half scale ones, are made as if I would be using them.
Mane hair makes a good cinch material for a couple of reasons. First, it does not stretch. Mohair cinches will stretch 1" to 2". If you're trail riding, expect a mohair cinch to stretch 1" or so. If you're doing any roping, especially big cattle, expect a mohair cinch to stretch at least 2". This is not necessarily a bad thing, but it is if you don't know your cinch is going to stretch and end up with one too long to use. There is none of this stretch in a mane hair cinch.
The other reason to use mane hair for cinches is the ability of the fiber to breathe. Because mane hair fibers are larger in diameter than mohair, there is more microscopic air spaces between the fibers as they are twisted together. Mane hair can't pack together as tightly in the twists, so more air gets through to the horse. They will also dry out faster than a mohair or mohair blend cinch.
Just like mane hair mecates, mane hair cinches have little prickles on them. These will wear off, just like those on a mecate. However, you must take into consideration that, unlike a mecate, you are putting this on your horse's belly and are pulling it tight. Not a good idea in the middle of summer when your horse has a smooth hair coat and thus no protection from the prickles.
Mane hair cinches require a breaking in period. This is best done in late winter or early spring when your horse has a very thick hair coat and is maybe even starting to slip a little hair. That is the perfect time to start breaking in a new cinch. Here's why. The goal is to wear off as many hair prickles as possible without irritating your horse. His long winter hair will give him this protection. The winter hair he is losing will come off in the cinch. This packed-in hair will protect his belly from the prickles during the summer until they all eventually wear off. It is not a bad idea to take the winter hair out of your curry comb and also lay this in the cinch. It will pack in as you use it and give your horse a little extra padding.