It's the rare mount from the murloc trader mini-game in Nazjatar. You'll need a hungry taco and the benthic cloak that lets you access the trader's special stock to see and buy it.
The mount only appears as a 'secret stock' item on a very rare basis.
The guide that explains how it all works and how to get the mount: https://www.wowhead.com/guides/mrrls...n-tidestallion
I am not sure if the mount is up in other regions too. I can only confirm it's up in EU because I'm stood here talking to Mrrl.
EU players, come and get your rare mount today!