Superiority of the small

Make use of you're size. Climb onto creature to attack them. You must be of smaller size then the creature your are attacking. While mounted your weapon must be one handed. On your turn you can use your action to climb on a enemy. your size must be at least half of the targeted creature. The creature most make a DEX saving throw to try and get you off. If it succeed you must make an acrobatic check to not get prone. If you miss  your action you take an opportunity attack. Once the creature mounted, you gain advantage on your attack roll, also you're attacks gain a bleeding effect (if the weapon you use is ether piercing or slashing dmg). On it's turn the creature will receive half of the dmg you did on your turn (it cannot stack dmg). If the creature is only one size bigger then you, it gets disadvantage on it's action except for it's saving throw to get you off. if the creature is large or bigger it can chouse between STR and DEX for its saving throw against you. if the creature is large of bigger it becomes difficult terrain when mounted. being on its leg or arm gives the creature advantage on it's saving throw.