The first horse I trained made every other horse easier. The reason was not that he was that awful, or that unruly, but rather that he was 12 years old and had had many riders prior to me. When asked to canter, he had learned to spin 180 degrees and RUN to dislodge a rider. He had also become hard in the mouth, just to make the days tolerable I suppose. Spartacus had been a 'reject' trail horse at a public riding facility, but to me, he was just BEAUTIFUL, and the answer to my 14 year old dreams.
The first day I owned him, I fell off him twice. Partially due to the 180 spin maneuver he had perfected, and partially due to the fact that I had money to buy a horse OR a saddle. So, I learned bareback, with a horse that I couldn't really screw up, and luckily lived to tell about it. Besides the spinning and running, he was a really sweet horse. Great ground manners, never bucked, kicked, or ever had a mean thought, as far as I could tell. I'm sure the instinct to spin and run was learned because he was 'trained' by trail riders who wanted to get a thrill, and the most for their money. And the spin, well, it was always in the direction of the barn or gate; He really just wanted to go somewhere safe.
From day one, my efforts were spent undoing what was done, and teaching him new behaviors. I once heard horse trainer and clinician Richard Shrake describe the training process as filling a jar with marbles. The white marbles were the good/desirable behaviors, and the black ones were the undesirable/bad ones. When training your horse, you have to put in more white than black. That is an incredible analogy, because it explains why it is so much harder to retrain a horse, rather than to start a young horse off right in the first place. AND, why it is important for a novice rider not to buy a young horse and think they can grow together. With every wrong move on your part, and every misbehavior that goes uncorrected, your horse is being trained. Badly. A horse that has just been with a professional trainer, has been filled with white marbles for the previous 30 days. If the rider doesn't know how to correct the horse when needed, it is a matter of time before the black marbles start accumulating.
I was lucky that, besides the spinning and running when asked to canter, Spartacus was a safe horse. Not easy to train, but at least I wasn't dealing with kicking, biting, rearing, bucking, and so on. I certainly would have been safer with a horse that had been trained and shown. One that had been there, done that, for the last 12 years. But, I wouldn't have learned as much!