We desperately need help. We own a 7 year old, 15.3hh, Heavy Cob. He's been my young sister's riding horse for the last two years. We got him from a woman who allowed him to do what ever he pleased. We knew this when we took him on but were sure we could fix his behavior. In the beginning, we put his odd moments down to babyish behaviour as his was only five. Now he is seven and his behaviour has changed dramatically. It started on the ground when he would bolt when we led him to the field. It then progressed to his ridden work: he started bolting and rearing. In the last week his behaviour has become more aggressive: he rears and strikes at his handler. We cannot catch him in the field as he will turn on you. Why has a nice placid cob become so nasty? We have always disciplined him when he has shown bad behaviour and praised for good. Now he will turn on you when you discipline him. He is going to be vetted but he is a very fit and healthy horse so I don't think it would be anything medical.
I'm glad you plan on having a vet see this horse, and that's something that needs to be done ASAP. Although he seems fit and healthy, he may have health problems you haven't noticed. Blindness, pain and dental problems can all cause bad behavior. Those conditions may be subtle and something you could miss. A thorough vet exam can help rule out and physical causes of bad behavior or recommend treatment if any are discovered.
After he has been vetted and either cleared of any health problems or treated for health problems that exist, you need to get him to a trainer. His behavior is escalating and someone is likely to get hurt if you continue as you have. He sounds like a spoiled young horse who has learned he can intimidate people, and however you are disciplining him, it is not working. Rearing and striking are two of the most dangerous behaviors a horse can display - they can get you badly hurt or killed.
I would talk to local trainers, explain his history and current behavior, talk about how you have disciplined him and how he reacts, and then send him to someone who has experience with problem horses and is willing to work with him. He sounds like he will need a trainer who is firm and consistent without being overly harsh. He's going to need a trainer who is attentive and able to keep up with him.
Once the trainer has worked with him and seen improvement in his behavior, you, your sister and anyone else who plans to handle this horse need to take lessons with the trainer to learn how to properly handle him and discipline him.
If this horse is a stallion, I also suggest getting him gelded. Unless you have experience handling stallions, the proper facilities to house a stallion and plans to promote him, you don't need to own a stallion. Stallions take time, attention and consistency and are best left to people who have experience with them.
Good luck with your horse. Please get professional help ASAP so no one gets hurt by this horse.