Seeing that no one is answering, I'll try (I've met them and my friend once tried to deal with one of them).
A stallion is not a horse for a beginner, not at all.
But even if we omit the rules of behavior with this animal, the owner's skills, etc. The biggest problem will be precisely that it is a STALLION.
90% of stables with boarding will refuse to board him because no one needs problems, and those who agree will set such a price that the idea of castrating him will become more and more attractive every day.
Also, do not forget that if this creature smells a mare in heat, it will break through all the fences and doors until it either kills itself or gets to her (for the stallion to be adequate, a VERY VERY EXPERIENCED owner is required) and then the game of roulette begins in terms of the reaction of the mare's owner and how purebred she was and whether the stallion harmed her (the stallion can also get hurt), but a headache and possible lawsuits and embezzlement are guaranteed.
Let's continue further, a stallion- kitten that was bought for crazy money in the hands of an inexperienced owner can turn into a monster in half a year.
Also, given the factor that stallions are prone to breaking through fences and doors, you should be prepared for the fact that he will stand in the stall 24/7, or walk alone in the distant paddock, so that neither he nor anyone else sees him, which will add points to speed up the process of his madness. The list can go on and on if you go into depth about training, maintenance, etc.
Therefore, I can advise that if you really want a horse with a penis, then it’s better to buy yourself a gelding that was castrated late, he will behave almost like a stallion, but he will be calmer.