Pandaren hunters automatically know how to tame Cloud Serpents. Hunters of other races must learn the skill from the tome How to School Your Serpent, which may purchased from San Redscale at The Arboretum in The Jade Forest. You require Exalted standing with The Order of the Cloud Serpent to read this tome, but the learned skill is account-wide. You can even purchase and read this tome on a non-hunter character.
This serpent is only accessible when you're on the quest Surprise Attack!. To unlock this once-off quest you have to reach Exalted with Shado-Pan and Revered with Golden Lotus, and then complete the precursor quest, Mogu Incursions.
Level scaling: In Shadowlands most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.