during a campaign or after a campaign, your dungeon mast can grant this ability to you.

Rider. you now have a rider that is is as big and as small as you. the rider you have can be any creature that can ride a steed. any ranged damage your rider does to a target is equal to (1d8) damage. any melee damage your rider does to a target is (2d6 + your strength modifier). when your rider attacks a target you use one of your actions to command your rider to attack that target. you can dismiss or call your rider at will. when you dismiss your rider, your rider leaves the area. when you call your rider, your rider comes to you until it is next to you. when you die, your rider doesn't die.

Note: non steed type characters cannot take this feat