I recently drew up these rules for one of my players who wanted to train a pet owlbear (as is suggested in its Monster Manual entry). Thought it might be worth sharing.
You can find a printable .pdf copy here.
You can spend time between adventures training an animal or monster to obey your commands. This downtime activity requires adequate space for the creature (5 square feet for a Tiny creature, plus 10 additional square feet for each size category above Tiny), enough feed to keep it healthy, and any other materials required to conduct the training itself. These costs are all factored in the overall cost, described below.
The creature you train must have an Intelligence of 5 or lower (minimum of 1) and have a challenge rating less than your character level. Often such creatures are beasts, but a few rarer creatures can be trained, such as basilisks, chuuls, ogres, and owlbears. Constructs, oozes, plants, and undead cannot be trained. If a creature that previously received training becomes ineligible, it loses all past training results.
To adequately care for the creature, you must carry expenses equivalent to a poor lifestyle (2 sp per day) in addition to your own. A Tiny creature usually only requires expenses equivalent to a squalid lifestyle (1 sp per day). This cost must be paid every day the creature is in your charge, even if you are not actively training it. If you fail to pay the cost for three days in a row, the creature escapes and all its prior training is lost. After an escape, it has a percent chance of dying equal to the total number of days you have spent training it.
After you spend a number of consecutive days working with the creature, roll on the Training Result chart below. Add the following factors to your roll:
- +1 for each consecutive day you spent training
- The creature’s Intelligence score
- Your Charisma modifier
- If you have proficiency with Animal Handling, you may add your proficiency bonus
Once you have added any of the above factors, subtract the creature’s challenge rating from your roll to determine your final Training Result.
If you train with the creature again, you can add half of this number (rounded down) to the next Training Result roll you make. This bonus persists as long as you spend 1 hour each day to reinforce the training. Otherwise, it ends after 1 month.