The general rule is, if it is bigger than you, it will see you as something to hunt, but if it is smaller than it will not willingly engage, unless it is part of a swarm that collectively has more mass than you do.
Toma's Method For Taming
The only units shown capable of taming ferals are Commander type units, typically warlords.
In addition, seafarer warlords have been shown to be incapable of taming ferals. Likewise, the only time we have seen a flying warlord tame a feral is Vinnie taming a bat, which vampires have a special connection to.
It's my belief that the seafarer and flying specials replace the ability to properly ride mounts, as I have never seen a seafarer or flying unit riding a mount before, even when doing so would be practical, for, whatever reason there seems to be a rule that forbids this from happening.
Therefore, I rule that all commanders that lack a flying or seafarer special are capable of taming.
The process of taming is simple. All you have to do is feed the feral while not damaging it. If the feral gets hurt, the effort of feeding it will go to waste.
It is also very important that the feral sees you offering the food. If the food is just lying on the ground, the feral will not associate the meal with you having provided it. It is also important to make sure that you choose the proper type of food. If you tame a dragon, you need meat do to do, trying to offer bread will only get the dwagon to ignore it and attempt to eat you instead.
Once the feral has eaten the required amount of food, amount depends on size, it will then turn to your side and become a proper unit.
Sometimes, feeding a feral isn't enough, however. In these cases, the feral goes into a friendly state that is in between tamed and feral. In this state, it will obey orders given by the unit that fed it, and move on the same turn as the rest of the side. This will continue until the start of his side's next turn. At this point, it will revert to it's previous behavior, unless it is fed again.