You must learn a special skill to tame Cloud Serpents, which is different to the skill required to tame other Lesser Dragonkin. Pandaren hunters gain the skill automatically, but hunters of other races must learn it 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.
Location & Notes:
Located in Mount Neverest, Kun-Lai Summit. The Wild Onyx Serpent circles Mount Neverest in Kun-Lai. It can be difficult to reach but you can just get in range of it if you find the right outcropping to stand on. At max level, use an item level 1 weapon (can be bought from a weapon vendor), remove your armor, and perhaps even kill yourself and take resurrection sickness to pull the serpent without killing it.