A horse that hasn’t been exposed to any sort of training — and that could even be just watching other horses get worked — or even just haltered can sometimes have a sense of fear because they don’t know what being trained and worked with looks like. This can be especially the case if they haven’t had a kind hand to work with them in the past.
Even if we do everything to make sure they have a safe, loving, and peaceful setup to be worked in, a lot of times they still go back to their defense mechanism that can be anything such as flight or fight because they haven’t been in a environment before that they don’t have to use them.
You can be doing everything you can, but sometimes you have to put yourself in their shoes and take a moment to think where the fear they have is coming from and work on things one thing at a time. Taking things at their pace and having patience for them sets in place that not just for them but also for us that you don’t need to force things to see progress.
If a horse is struggling with being aggressive, try to find out why exactly they are that way instead of getting mad at them for it. Horses are a lot smarter then we often think, and since they can feel our heartbeat from six feet away, they can sense if we have faith in them that not just they can overcome the feeling that they need to constantly be on guard, but also come out of that state of fear as a partnership with faith.