When we got our TB gelding, he was five yrs. old and fresh off the track (3 months)..
...and while not quite aggressive as yours, he had severe issues. When he didn't get his way, he would go crazy.....like if he wanted to go out to the grazing pasture but it was being used and he had to wait....or if it was a windy day and we were trying to hand graze him......he would whirl, buck and rear...and I mean a strike out with his front hooves in your face rear.
I was terrified he was going to kill my 20 yr old son, who is his owner...and who had the responsibility of training him. Now, my son is no professional by any stretch of the imagination.....but he had 4 yrs of Parelli training and a bit of richard shrake training....and I was totally convinced he was in waaaay over his head.
But he is an adult. It was his horse, and his choice whether to try to rehab him or not.
Here we are two years later, and our OTTB has come such a long way. He is a delightful, well behaved boy who is just a big gray love bug. He has made great progress in learning to control his temper, impatience, horse on horse aggressiveness and his impulses. It is truly amazing to see a horse like him transform into self control and maturity.
So, retraining these horses can be done....but it takes courage and commitment.
OTTBs aren't taught manners at the track.....likely what you are seeing in your horse is the same as we saw in ours, only a bit more extreme in your case.....you are seeing an emotionally and socially immature horse with no idea of how to control himself....
When my son first introduced our OTTB to the Parelli 7 games training, he bit the carrot stick and tried to kick at it....now, he can do all the Parelli exercises without the least resistance or stress....he is a trainer's ground work/lunging dream.
Obedient and respectful.
Under saddle he is VERY forward.... BUT as long as you do not carry a riding crop, he is obedient and very eager to please. Bring a crop with you, even if you don't use it, and he is a nightmare of bucks and rears. Guess it's post traumatic racing syndrome or something equivalent