I have a 16 year old Morgan that we bought in September 2009, didn't want to do too much with him when we got him because he was underweight, although we did saddle him and I had my 16 year old get on him to see how he would do, he did good for her, so I tried to get on and he tried to lay down.
After that we let him have the time to get the weight on and he is coming along nice so for the last couple of weeks we have been working with him, a couple weeks ago my 13 year old got on him while he was tied so I could adjust stirrup length and he tried to lay down, then we brought him into the pasture to ride and he tried to lay down again. He doesn't actually get all the way down, but I got him to stop and then she rode him fine. For the last 2 weeks my 16 year old has been doing really good with him bareback, we thought maybe it was something to do with the saddle but tonight when she went to get on he did the same thing, she ended up falling off twice. He did have his feet trimmed yesterday so I don't know if his feet are sore although he tried doing it a couple weeks ago, so thinking it's not that. Our farrier did tell me 2 trimmings ago that it looks like he foundered in the past, have never dealt with a horse that has foundered.
Anyone else ever have a horse that does this?? How do you get them over it while keeping the rider safe? Could it be his way of saying he doesn't want to be ridden?
I got mine underweight as well, 500lbs underweight, and when I was able to start getting on him I had some what of the same issue, but he would sit down. I palpitated his back and with his body ever changing his back was sore and off. Also appling weight to his spine was painful as well. I started to do some stretched for him to build his topline and tummy to better hold my weight and now the only issue we have is standing still when I mount!
Maybe his back is sore and needs to be adjusted. For stretches there were two of them that I did.
1- put both hands at the girth area on his tummy facing his withers and start to "tickle" him. As you see his back start to arch upward, then move the hand closes to the hind end up his tummy to just before the sheath area and tickle there. Only do it in 30 second increments. After he starts building muscle you can just start where your second hand ends as he's probably built enough muscle and knows the drill. I have seen a DRASTIC change in this.
2- From behind, place both for-fingers at the top of his hips, one on each side, and slowly "tickle" him as you move your hands down his hind end toward his tail. You should notice him lifting himself up and his butt kind of sticking up. Again only do in 30 second increments. Eventually you will just be able to apply pressure to each side of his hind end on either side of his tail and he will stretch for you.
And I always finish with carrot stretches!!!
Hope this helps and good luck!