Bought a brand new mohair girth and ride it with my brand new MJ Lightspeed and ended up on the ground. Apparently my nylon billet and the mohair cinch has a lot of stretch. Anything I can do to cure it?

Use it,  With use it will stretch to its maximum stretch.  Washing will cause it to shink up some but with use it stretchs back.

The nylon billets dont stretch/give, its your mohair cinch that did all the stretching, did you get the same size mohair cinch as your other cinchs? If so you should have gotton one that's a size down/shorter since the mohairs stretch quite a bit, they can stretch as much as 2 inchs, knew a few people that would order a mohair in the same size as their old cinch and it never worked out, they would stretch out   almost 2 inchs and caused a few wrecks. When I ordered one I went down a size, I should have gone down 2 sizes sinch it stretch more then I liked. So I just went back to my PC fleece and didnt get that smaller size. I bought the mohair to just want to try it out, but decided I like my PC and Classic Equine's better anyway. 

True this! I used to use a 34" typical girth but had to go to  32" in mohair. You always have to tighten again before heading down the alley!

I use nylon billet and latigo and the mohair girth now. I used to have nylon billet and leather latigo and that was too much give.

Just echoing, mohair can stretch a couple inches and it will shrink some but will need to be restretched after a wash even though not as much as the initial stretch. 

Dang it. What have I done? 

Dang it. What have I done? 

I have 4 mohair cinches from 5 Star and I ordered the same size as my regular cinch with no issues.  I noticed them stretching a tad after I used them a few times, but nothing as drastic as two inches.  I'm talking maybe one hole on your latigo.

I don't know if I just got lucky, or if 5 Star does something in their process to "pre-stretch"', so to speak.   I know they have great service if anyone wanted to call and ask before placing an order.

I thought about getting a 5 Star cinch but I didnt want to spend that much money on a cinch since I didnt know if I/horses would like the Mohair, so just went with the Classic Equine just to see what they were all about, mine did stretch and should have gone with 2 sizes smaller, but after using the mohair that I got I decided to just stay with my fleece cinches (PC and Classic Equine) the first mohair was just a test, lol

5 Star is such a great company and I have heard that their Mohair cinches are really nice, and made so much better then the rest, I have many pads of their's in different lenghts and thickness and these are the only pad's I use. I have looked at their cinches on their Facebook and website and oh man they are so pretty and I know they are worth the price but Ouch!!!! 5 Star use the best products out there so I know their Mohair chiches are Pure Mohair so they should not stretch much at all, and thats why they cost alot more, the other companys will use a blended mohair with wool, so thats why they stretch and are cheaper. But for my first time I just wanted to test the water. LOL..

What brand did you get? I got the Classic Equine Mohair and its 100% percent mohair but it still did some stretching. They say pure mohair dont stretch very much but mine still did, not as bad as other brands. You just got to be carefull and really read between the lines when buying a mohair I have learned. 

I prefer the alpaca over mohair, doesn't seem to stretch nearly as much. I can use the same size alpaca as I do a fleece or neoprene. I also use mine with leather latigos on both sides, I hate billets. 

Red Mare Mohair. 100% mohair. Very nice cinch. I just didn't know about the stretch. Maybe mohair should come with a warning. Ouch.

How do you like the saddle aside from ending up in the dirt?

Jury is still out. I haven't ridden it enough to know.