Lustrous Glimmershell
Quest reward from Glittering Shell. A gift, found one day and saved for a meaningful occasion, should things ever turn around. Aquatic type. Each of the Nazjatar followers on Horde and Alliance grants an item once they reach level 20.
The 
Lustrous Glimmershell is a Horde-exclusive pet obtained as a gift from the follower Poen Gillbrack in Nazjatar once he reaches level 20.
Guide awaiting review
Level Poen Gillbrack to 20
As a Horde character, level your Nazjatar follower Poen Gillbrack to level 20. Once he reaches level 20, a quest will become available from Poen himself — accept it to receive the Lustrous Glimmershell as his gift.
Summon Poen Before Visiting Vendor
If Poen is dismissed when he hits level 20, the quest will not appear at Atolia Seapearl at 50.8, 65.2 in Nazjatar — summon Poen back first, accept the quest from him, and only then will Atolia offer a second copy for 
40 Prismatic Manapearls if you want an additional one.
Alliance Players Need Separate Grind
This pet is not cageable and is not awarded cross-faction — Alliance players must level Bladesman Inowari to 20 on an Alliance character to obtain their equivalent pet, and Horde players who want the Lustrous Glimmershell must level Poen Gillbrack on a Horde character.
Faction Changed? Buy From Vendor
If your character already completed level 20 with the respective follower on a previous faction before a faction change, you can purchase the missing pet directly from Atolia Seapearl at 50.8, 65.2 in Nazjatar for 40 Prismatic Manapearls.
Tips
The Mount and Toy rewards from Nazjatar followers are shared cross-faction upon learning, but the Lustrous Glimmershell pet is not — it requires leveling the Horde follower Poen Gillbrack specifically.
