It's common to label a horse like this as aggressive, antisocial, dominant.
There are actually many factors involved in Flynn's behaviour. Out in the field with the others he looked very relaxed. He needs to have a good bit of space to feel he can safely be around other horses. So much of his behaviour was about maintaining a safe space around himself. This may have come from being cornered/injured/threatened by other horses in the past - we don't know.
If we label an animal, we tend to lose empathy for them, and we may be annoyed or frustrated by their behaviour.
Instead, we could consider this behaviour as a symptom of an underlying problem; his well-being is compromised. It could be a sign of pain, fear; really any form of stress. We can then consider how to help him.
Having sufficient space and feeling safe are basic needs for all animals, and vary for each individual. In this case, we could simply say that his basic needs are not being met in this environment, causing stress which then (as is often the case) causes aggression.
Flynn will also behave more aggressively than usual if other basic needs are not being met; for example if he is cold and wet, or in pain.
Even if we can't resolve the issue entirely (we didn't move to the new environment in the video for over 5 years), we can at least make some changes to improve things, and appreciate that we should be looking for ways to help this horse rather than impugning his good character.