The last one was for the 8.0 (2018 - 3 years ago). Now we had many other mounts (more than 400) and with the 9.2 we will have many more other ones (40 mounts).
It could be nice to add new achievements for mount collection (and pet collection too).
Please don’t add re-skins mounts for these new achievements. For these ones, you can add “new” and cool mounts and not just a re-skins mount (Red Dragonhawk or Frenzied Feltalon for example).
Sure it takes time to make new mounts. But there are already some “new” and cool mounts in the game that you could use for these new achievements. These models are already done so why not to use them for that. Even with just a saddle or an armor on it, I’m pretty sure some of these models would be great for the new mount collection achievements.
Here are the models I have in my mind:
Eagle 
Felbat 
Owlcat 
Nerubian Swarmer 
Stag 
Saurolisk 
Ravasaur 
Ravasaur (other color)
Rugged Silithids 
Rugged Silithids (other color)
All these ones (or some) would be great for the new mount collection achievements.
Thank you. 