Crimson Tidestallion

The blood-red coloration of these creatures is ideal for hiding in large coral formations.

Riding Requirements:

This mount is available to all eligible characters on your account.
Level 10
Apprentice Riding

Source:

Mrrl in Nazjatar.
Cost: 4 2 1

Notes:

This mount is sold infrequently by Mrrl, a murloc vendor at your Nazjatar faction hub, as part of a complex bartering system. Here's how to purchase one:

Complete the intro quests for Nazjatar to the point where you unlock world quests.
Find the wounded murloc - Mrrl in the north of Dragon's Teeth Basin and accept its rescue quest (A Safer Place / A Safer Place) and bring it to your faction hub - Newhome for Horde, or Mezzamere for Alliance.
Upon completing the quest, four more vendor murlocs will appear around your hub: Grrmrlg, Hurlgrl, Mrrglrlr, and Flrgrrl. Mrrl offers you one more quest to buy four items from the other murlocs.
After you complete the final quest, Mrrl also becomes a vendor. The item(s) it offers may be purchased using items bought from the other murlocs, which in turns require other items sold by the same murlocs, i.e. a kind of bartering game.
The items Mrrl sells change daily so you'll need to be patient and check back every day. In the meantime there are other things you need to do:
Obtain an Azsh'iri Stormsurger Cape. These are created (randomly) from Benthic Cloak tokens which can be purchased from the benthic gear "Vaultkeeper" in your hub for 5 Prismatic Manapearls. Depending on how lucky you are it may take a few tries. You can earn more Prismatic Manapearls by completing various daily / world quests. You won't see the Crimson Tidestallion for sale on Mrrl unless you're wearing this cape.
Two of the three epic items required as currency for the Crimson Tidestallion (see 'Cost', above) can be purchased from the other murloc vendors around your hub, but only when you're wearing your Azsh'iri Stormsurger Cape. Otherwise the items are hidden. These items have a 1-day duration so don't purchase them until the day the mount is on sale!
The third currency item you need is a Hungry Herald's Tentacle Taco. This is sold by Murloco, a murloc who appears infrequently in Murloco's Hideaway in the north-west of Nazjatar (his cave is roughly due west of the "A" in "Azshari Terrace" and due south of the second "E" of Queen in "Gate of the Queen" on the zone map). You may need to camp his cave for some time before he appears. You may also need to kill some naga guards to free him. Fortunately his item doesn't have a finite duration, so it can be purchased days in advance.
If you don't want to work out the bartering game for yourself, here's what you need to buy:
Round 1: purchase 288 Unidentified Mass from Flrgrrl; 128 Sweet Sea Vegetable from Hurlgrl; 714 Slimy Naga Eyeball from Mrrglrlr. This will cost you 1130 gold.
Round 2: using the items you just bought, purchase 238 Curious Murloc Horn from Grrmrlg; 32 Just Regular Butter from Flrgrrl; 48 Dirty Murloc Socks from Hurlgrl
Take the 48 Dirty Murloc Socks to a water source and wash them (click on them till they're all Clean Murloc Socks).
Round 3: using the items you just bought, purchase 16 Sea Giant Foot Dust from Grrmrlg; 28 Ghost Food from Flrgrrl; 14 Healthy Murloc Lunch from Hurlgrl; 16 Smelly Pile of Gloop from Mrrglrlr
Round 4: using the items you just bought, purchase 2 Pulsating Blood Stone from Mrrglrlr; 4 Cultist Pinky Finger from Hurlgrl
Round 5: using the items you just bought plus the Hungry Herald's Tentacle Taco, purchase the Crimson Tidestallion from Mrrl!