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.
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.