War Whisperer

You can spend a week of downtime to train a beast for combat. At the end of the week, make a Wisdom (Animal Handling) check, DC determined by the DM. If you succeed, the beast:

Will accept a mount, if it is physically capable of doing so.
Will not flee or charge recklessly into combat. It will instead focus on protecting you and itself.
Will follow the "Attack" and "Disengage" commands given to it. (Issuing a command is an action.)

If you fail, you can spend another week of downtime to try again.

Wild animals must be tamed before this training. You can spend a week of downtime to tame a wild animal. At the end of the week, make a Wisdom (Animal Handling) check, DC determined by the DM. If you succeed, the beast no longer treats you as hostile and can be trained for combat.

The CR of the beast may not exceed ½ your character level.