"You come from a lineage of beast tamers who rid their mounts into battle, as such whenever you hop on top of something you feel... At ease, you are more adept to riding on creatures then most other beings in this realm, ride your beast to glory.
You're used to riding random shit in your backyard, you can ride any beast type creature one size larger then you assuming you have its permission.
The power of friendship and food:
You can talk with animals, no not literally but you're really good at guessing what they're thinking.
At 1st level you gain access to the spell Animal friendship at 1st level that you can use one time without consuming a spell slot, this usage restores upon a short rest, at 6th level you gain the ability to gain the Dominate beast spell in the same way, and if you use the Dominate beast spell in this way once every day for a week, The beast will be bonded to you like a weapon or item, yes this bond uses an attunement slot.
You ride things, a lot.
Whenever you are riding on a creature you gain advantage on all attack rolls, and gain a reach of 10 feet on melee attacks.
You also gain an extra attack feature like fighter/monk, this stacks with other extra attack features.
When you are riding on an animal and are attacking another creature that is of the same type as the one you are riding on, ie horses if they're a horse that creature has disadvantage on all saving throws against attacks from you, It also takes double damage from attacks from the mounted creature.