When I say realistically obtainable, I mean if someone new to the game started collecting mounts, if they put in enough effort, and didn’t have RNG working against them, they could get it.
AKA If they got every single raid mount on their first try, got any pvp mount they could rather fast (I don’t know how saddles work anymore, I gave up trying to figure that out long ago) etc. In other words, if they put the maximum amount of effort in, and got total luck on their side they could achieve it.
Because if there’s an achievement you can’t get no matter how much effort you put in, until a later patch/expansion, that’s demoralizing and may turn someone off from collecting or the game itself.
Like, for instance, if they added a 1500 pet achievement in 9.2.5 without adding any new pets. Then only people who have old unobtainable pets would be able to get it until next xpac. That would feel horrible and actively make me lose interest in collecting.
They did this with Hearthy, in which they added the 1,000 pet achievement before it was realistically possible, in a pre-patch. I personally witnessed multiple people in WCP just outright stop collecting over it. I don’t know if any quit the game, but I know of at least two that stopped collecting.
According to Enhanced Mount Journal, I’m at 483/783 of mounts my character can use (I’m not a mount collector. I just… somehow accumulate them? lol). So I don’t think 800 is technically realistically achievable. It IS achievable, just not realistically.
For instance, dataforazeroth has a ton of people over 800, but it has a disclaimer that it counts both Horde and Alliance mounts and class specific mounts. And as the in-game achievements require a single character being able to use them, the vast majority of those people wouldn’t get an 800 achievement. And there’s no way to separate that on their interface. Whereas the achievements discord has like 2 people with over 800 (and that tracks per character). There’s less than 50 with more than 700 on there, lol. And there’s like 50,000 members or something like that, and a good majority of collectors are on there.
For the record, as someone with (currently) 1447 unique pets (no protoform pets yet), I would be against adding say, a 1450 unique pets achievement in 9.2.5, for instance, because a new person realistically can’t reach that, as I have a lot of unobtainable pets.
So trust me when I say I understand wanting an achievement for your accomplishment. I just personally wouldn’t want to demoralize and make another person lose interest in collecting or the game in general, because there’s an achievement they can’t obtain for an expansion or two down the line because I have unobtainable things.
All that said, I wouldn’t scream or anything if they added them, I’d just be disappointed. You wanting them is perfectly valid. I don’t want this to come off as dismissing your desire. That’s not my intent at all. Just trying to point out that if an achievement isn’t realistically obtainable for patches/expansions due to missing old content, it’s demoralizing and discouraging to people.
And if it wasn’t obvious, I’m VERY MUCH in favor of adding new mount/pet/toy achievements. 
Just to realistically achievable numbers.