I frequently do consultations with people regarding their horses, facilities, planning, and basically anything horse-related you can think of.
I recently went to a private farm where the people have been having a lot of aggression issues from their horses. They have a very alpha mare and a gelding. The gelding gets picked on a lot by the alpha mare. That is a very normal thing. The gelding has developed some serious aggression issues. He charges, bites, pins, rears and kicks. He has no respect of personal space at all.
The mare gets jealous of any attention the gelding receives and will circle you like a vulture and pin her ears. This is a very dangerous situation. They are scared of the horses, and the horses have picked up on this. The gelding essentially bullies the owners just like the mare bullies him. They do not stand up to the horse and make him behave. They work around him timidly, and they even carry a stick with them for the horse to bite on, much like a German Shepherd would bite onto a bite suit.
I have never seen anything like this in my life! I told them that I would agree to work with these horses if they came to my farm, and that it would definitely not be a 30 day process. This is an extremely dangerous situation. I am actually very worried for the owners. I know that I can help the horses, but so often the owners are not willing to spend the time and money to fix the horse. They are the ones who ruined the horse.
Please always remember that every interaction you have with a horse will either teach the horse in a positive or negative way. Far too often, I see people negatively condition their horses just from their daily handling techniques. You have a responsibility to the horse. Please do right by them!