Mounted. During a long rest, the wyvern can designate one creature that rode it within the last 10 days as its rider. The wyvern gains temporary hit points equal to 3 times the rider’s level or challenge rating after completing the long rest. The wyvern uses its rider’s proficiency bonus in place of its own proficiency bonus for AC, ability checks, attack rolls, saving throws, and the DC made to save against certain action effects.

Armor Class. 13 + PB

Temporary Hit. Points 4 times the rider’s character level or challenge rating

Saving Throws. Dex +PB 

Skills. Perception +1 + PB

Intimidating Presence. The rider has advantage on intimidation checks made while with 10 feet of the adolescent wyvern.

Evasive. The wyvern can move up to half of it's speed as a reaction when an enemy ends its turn within 5 feet of them. This movement doesn’t provoke opportunity attacks.

Claws. Melee Weapon Attack:+4 + PB to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) slashing damage. On a hit the target creature is wounded and at the start of their next turn they make a DC 12 + PB Constitution saving throw. On a failed roll they take 2d8 slashing damage

Bite (3/Day). Melee Weapon Attack: +4 + PB to hit, reach 10 ft., one creature. Hit: 11 (2d6 + 4) piercing damage, and the target is grappled (escape DC 12 + PB). Until this grapple ends, the target is restrained. If the target is Medium or smaller, the wyvern can drag or carry the target while grappling it without any movement penalty.

Stinger (1/Day). Melee Weapon Attack: +4 + PB to hit, reach 10 ft., one creature. Hit: 11 (2d6 + 4) piercing damage and make a DC 11 + PB Constitution saving throw. On a failed save they taking 7d6 poison damage and become poisoned, or half as much damage on a successful save. A poisoned creature can repeat the save at the end of their turn to stop the effect.

Weave. When the wyvern has to make a Dexterity saving throw, the rider can use its reaction to grant the wyvern advantage on its save.