Grotto Netherwing Drake is a potential sixth Dragonriding Drake that appeared in the latest Patch 10.1 PTR build. The current source says it’s from the Trading Post, but it’s most likely the 10.2 campaign drake.
In the latest 10.1 PTR build, Blizzard added a new Dragonriding mount! The tooltip currently mentions it will be available from the Trading Post and cost 999 Trader’s Tender, as discovered by MrGM, but this is probably just placeholder information.
If the mount were obtainable from the Trading Post, I would expect it to be Full Transformation instead a full-fledged mount, how would obtaining all the customization options work if it were only obtainable from the Trading Post? Anyway, this seems more like a new 10.2 campaign mount.