Doesn't like Being Girthed Up
When you place the saddle on the horse's back, if it doesn't fit well, the horse will feel little discomfort because there is no pressure on it. As soon as you start to tighten the girth those pressure points will start to be felt by your horse and will cause discomfort and pain to him or her. When you are about to tighten the girth the horse will associate this with pain and will do his or her best to stop it from happening. He may try to get away from you or he may kick out at you or try and bite you.
Use The Perfect Fit Saddle Fitting Kit to check if the saddle fits. If it doesn't, be sure you get one that does and don't ride him in the old saddle until you have. Using the kit, check the saddles's fit at least every month, or more often if there are changes to your horse's diet or exercise.
When you are certain that the saddle fits well, but the behaviour is maintained, it could be that your horse remembers the pain that he was in previously, or, he is still in pain from the damage done by that saddle. If this is the case, you must have his back checked by an equine physiotherapist, chiropractor or osteopath. In fact, if a horse exhibits any unpleasant behaviour, the first thing to do is have him checked out by one of these professionals.
Sometimes it can take a while for a horse to trust that the saddle isn't hurting him, so be patient with him if he takes a while to become relaxed in the process. Sometimes the upset that the pain causes him can create an unbalance of the healthy gut flora. This, in itself, is enough to cause your horse to be tense and exhibit undesirable behaviour. This unbalance can last a long time, so it is always worth giving a supplement of probiotics to a difficult horse.