That old Vashj'ir Seahorse

[Felfaadaern Darkterror]

Does it count for numerical mount achievements? Do you have to be in Vashj'ir (where it's "usable") for it to count? If I get to 299, should I go to Vashj'ir and mount up to see if I ding 300?

[daytonbrown]

It counts towards the achievement. That restriction is for when the mount can only be used by the other faction.

[Lord Pebbleton]

Learning it is enough to have it counted among your mounts. You don't need to be there and use it. This is also true for Qiraji mounts and class mounts that belong to one of your other characters. The only time it doesn't count while still being listed is when the mount belongs to the enemy faction.
If you have a somewhat high level paladin, you can try logging him. Paladins' class mounts only count when you are playing that character, and (excluding Legion ones) they have three.

[Miatog]

It counts towards the achievement. That restriction is for when the mount can only be used by the other faction.

Or another class.

[yetgdhfgh]

It does indeed just like the 4 AQ mounts you can get also class mounts count if you change characters.