@Title.
I’ve been stuck at 396/400 for some time now, since then I got 3 mounts (rajani warserpent, the bird from rotfeaster and the dog from houndlord) but the counter is still at 396/400.
Does anyone know if it’s bugged or if for some reason these mounts are not counting towards the achievement?
Isn’t it based on one character?in which case although lock and pally mounts only count if your on that character but not another? I’m not 100% on this just trying to help troubleshoot with ya.
i have a paladin just because of that xD
i even used it in the argent tournament, that’s my mount char, but getting these new bfa mounts hasn’t changed the counter on any of my characters, even that one
Odd then. I have had them not count until I hope oj my paladin but I also had a delay in mount achievements as well. Like I logged into my mage and randomly got mount parade and it’s been a few days since my last mojnt acquired. And the mage is an boost so no exclusive or class mounts of any type for him.
After reading a comment on wowhead from a dude saying that for some reason the mount # wasn’t updating properly on his paladin when getting mounts on other characters, only when getting them on the paladin itself, so I decided to give it a try, I went to Vashj’ir on my paladin and completed the questline unlock the seahorse and the achievement immediately jumped from 396 to 401 and I got my feltalon 
Wierd bugs lol but congrats and glad you got it sorted out.
Yes, I read that when doing mount collection:
The Paladin/whomever actually has the Unlock the Vash seahorse, for it to add to his mount number.
On this character, I also had it displayed weirdly when going for “Lord of the Reins” (300 mounts?) It said I had something like six left, but upon buying two from the Ah I was at 301 mounts and got the achievement. I’m not sure why it is like that.
Mount counts are screwed up in the Armory, achievement and under your mount tab. The only place to accurately track this number is under:
Statistics > Character > Gear > Mounts Owned
Check your counter here with your Paladin.
Please check my answer here Achievement not updating mount count.