I’m pretty sure I speak for a significant part of the mount collection community with this post.
It feels like a slap in the face when the store / sub promotions get sick new mounts, while some of the most dedicated players who have collected for years get something as dull as this.
And also, where are the 600 & 700 achievements?
Link to the WoWhead article:
Sigh… now I need to farm even more mounts 
it always been 50 mounts difference between each achievement why they jumped to 500 and left 450. plus i totally felt the same when i saw this mount compared to what the new promotion mounts look like. i guess there could be a better recolor mount atleast. this mount looks so basic that people will mistaken it with the minion of grumpus and the one from island exp.
Everything they adding is so outdated for sure, i sit on 800 mounts and they add 500 
It shoud be 1k mount achiv, who cares if nobody got it.
Lowest possible effort. Feels good being a mount collector.
Feels good to “not” collect mounts atm. The Yeti is the most boring mounts in game IMO!
Yetis are fine, but we already have those. Theyd better light it on fire and add a bunch of sparkling crystals before I pog over it.
Rather dull in comparison to almost any other mount. But as you say sparkling crystals and stuff like that would make it cool 
Strange that they didn’t choose to honor dragons atm. Or some cool protoform mount (ZM style)!
This is a pretty bad reward for hitting 500, i pray that it’s a placeholder and they didn’t just go ‘‘You’ve asked for years so here’s a yeti so you’ll never bother us again, next time it’ll be a lightly browner kodo’’
I like the look of the Secretive Frog Duck pet. Not many needed to get it.
I’m also almost at 500 mounts and I don’t want a yeti. I want something dark and mysterious and flying.
Imagine a Dark Firehawk. Maby a Black Storm Dragon? So much cool stuff they could do for 500.
Either of those would be excellent for a SP though possibly I’d prefer a black storm dragon to fit in with the theme of the expansion.
Wait, is that the remake of the mount you can get in AH ? 
That mount doesnt look good.
we want a Duck mount!
They don’t.
Every mount looks worse than the previous one.
I can agree that the water tiger When lo, looks “Decent” but it’s still not something i would use ever.
The newest one? the wyrm? Who would even look at that and say it’s a good mount?
Same opinion for me with pretty much all the mounts in the store, The mounts you can get ingame just look better.
They need to change this ugly butt no effort mount.
If they put even 20% effort into it as they would a store mount it would be fine.
Blizz are literally souless and whoever greenlights these decisions should maybe have
a think and wonder if they even understand their game or the playerbase.
No you do not, you are on 800 which is still a lot of mounts but why lie about another 100.
Most of them you look boosted to.
My reddit post is picking up some, help sharing this post so we can get this changed!
Yeah, please guys, change the reward and throw in an achievement for 450 mounts too. Not sure why you skipped straight over that. Anyway, onto the suggestions for different rewards:
RE-SKINS, IF YOU MUST INSIST:
DF’s coming up, so why not add the Mottled Drake and Feldrake from TCG for rewards? All those other mount should be in the game, and there’s no better time than the present!
Unarmored version of Sinister Gladiator’s Proto-Drake (us PVE players never got our version of the updated proto-drakes, so come onnnnn)
Following on from the above, ground soul eater. Keeps it unique, don’tcha think?
Stormcrow. This bad boy has been in the database for years. Dust it off and put it in action.
Green Stormdrake
Updated Silithid mount. The purple one from the database would be nice, please!
UNIQUE MOUNTS BECAUSE HEY, PET COLLECTORS ALREADY HAVE 2 (VENUS + CELESTIAL DRAGON)
Undead Maldraxxus drake like (read next bit as Grandmaster Vole, please) XANTUTH THE BLIGHTED!
Mount versions of Tempered Stygia and Stygian Bombarder.
Mount version of Sinstone golem.
BONUS FOR 550