We have owned this pony since early spring of his three year old year. He has always been calm and super easy going, lazy in fact. He is not one to act up, buck, rear, spook, or even shy. The only thing he had ever done was kick a little at spurs when they where first used on him. I was his main rider and show handler for three years. Each spring he had one-two months of professional training. His six year old year, I continued riding him, but a eight year old girl started riding him more regularly and showing him some. The following year (last summer), She was riding him and I was riding a young horse. We were both loping & she stopped and told me that he tried to kick at the fence. I said maybe he miss-stepped try again. They loped again & he stopped backed up and bucked three times. Then, proceed to do the same each time she asked to lope. I got on and he gave me one buck asking for the trot. The vet, looked at him finding only a slightly sore shoulder. We gave him some time off then went to light lounging. This spring she rode him in a lesson which went fine until midway through loping. He stopped and bucked like last year. This went on a few more times. The trainer had her swat him with the reins each time and they did manage to ride a little more. He then had him vet checked again and treated by a chiro and acupuncturist. The next time, I took him to ride in a lesson. Things were going fine also, a bit sluggish as usual. Until we were loping (hadn't reversed yet/in the left lead, first time this happened was right lead) & I felt him slowing down. A few strides later he was completly stopped and bucking higher than the previous times. He reluctantly went into the walk. Half a round later, I asked him to trot and he repeated the same behavior. I put him on the lounge line and he lope, but not willingly. Other info: He has had negative tests for EPM and Lymes. There have been some times that I noticed him tripping. We've been using the same tack we've always used. He has a weak immune system. He's lowest on in the pecking order. He has a displaced lower jaw & likes to chew on things. This has happened both indoors and outdoors. He never takes off when he does it just stops while loping and bucks from a standstill multiple times.
Some of this may not be related & if any other info would help, let me know. Thank you for your time & Thank you in advance for your help!