Arash-ethis is a level 36 rare white wind serpent that can be found patrolling in one small area in north western Feralas, just south of Camp Ataya. I have indicated where he can be found in red on the map below.
He drops the typical green item and a decent amount of experience for people leveling.
He is also tamable by hunters and would make a nice pet for people who come across him. He shares the same model and color as a few other mobs in game, the most easily obtainable being the level 40 Cloud Serpents in Thousand Needles. He also has a twin rare spawn in Dustwallow Marsh called Hayoc.
Arash-ethis can be found with NPCScan. You will need to add it, his NPCScan ID is 5349.
You can also use this macro to help find and tame him:
/cast Tame Beast
Before the Cataclysm his spawn timer was somewhere between 5-8 hours. I am pretty sure that just like most other rares since the shattering, it has been dramatically reduced. He was up the first time I went looking for him.