The Dark Ranger's Hood is a mail transmog item added in patch 7.1 for hunters. It can also be used by shamans, but only hunters can obtain it. In order to be able to get one, you need a level 101+ hunter that has accessed the Trueshot Lodge and has an Artifact Weapon.
If you meet the requirements, go to Trueshot Lodge and talk to Death Hunter Moorgoth inside the main building. He sells roses, but don't buy them yet. Once you have talked with him, go and find Dark Ranger Velonara in the same building and talk with her. Now head back to Moorgoth and but a total of 13 roses from him (it costs about 26g total). Talk to Velonara again, and once again talk with Moorgoth. This opens up a quest from Velonara (Dark Memento) which you end at Moorgoth again. Upon doing so you will be able to buy the Dark Ranger's Hood from him for about 20g.
Once you've bought it on a level 101+ character, all hunters and shamans on your account can transmog this item.