Have had Turtles All the Way Down achievement since 10/01/11 but when i came back from the release of BfA after a long break the Sea Turtle mount is missing.
There have been a lot of issues tonight and some things are not showing up correctly. Give it a little bit of time for the issues to be corrected and then check again. If the mount is still missing, then submitting a ticket would be the next step.
https://us.battle.net/support/en/help/contact/464/ticket
Be sure to reference your achievement that shows you had the mount.
My sea turtle is gone too. I used it with in the last month. Got achievement 5/15/09. Submitted a ticket and told nothing Blizz would do to help. Told me to reset UI. I have done all they suggested and turtle still is gone from the mount list. Others in my guild can see the turtle on their lists. I am going to try a bug report tomorrow. Anyone else have/ solve this problem?
Does the mount appear when you use this command in game?
/cast Sea Turtle
Also, ensure all your filters are correctly marked in the Mount Journal.
Give me a minute and I will see 
Oh my- that worked!! Of course I have logged out so I don’t know if that repopulated the list or not.
filters are OK - thank you =
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You’re welcome.
You can make it into a macro and put it on your bar if you continue to have issues summoning the mount from the Journal.
I will do that. Thank you so much!!! Stay well and safe. !! Mary
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You as well.