You mave have already read about this topic in the "general Discussion" Forum.

Below you will find the idea, if you haven't read about this topic before. 

Since I have no idea of modding I can't say how difficult this is, but I imagine that just giving these units (pegasus knights, longma Riders, pegaus/dark pegasus mounts, furries, terradons, riperdactlys (mounts)... the "hide in forest" passive ability should be enough to reach the goal).

Please comment down below, if you liked the idea and/ or going to mod it into the game. That would be awesome!

Idea:

 In my opinion, it is just very strange that longma riders and Pegasus knights cannot hide in forests when landed. It takes a lot of tactical depth out of the game and makes named units a lot less interesting than they could be. Also, it shouldn't be too hard to implement, so from my point of view this should be low low-effort, high-reward task. Sadly I have no idea of modding, so if anyone who knows modding likes this idea and knows how to implement it, please consider putting this feature into the game via a mod. This would be mind-blowing awesome. 

To be completely honest, when CA first announced, that flying units would be able to land on order, I thought, this mechanic would be instantly in the game and was quite disappointed when I realized it was not.  I really hope that this will be implemented at some point, the sooner, the better.

It's easy enough.  You just an attribute to the attribute groups these units use.

Yes, indeed. But as I said I have no experience with modding. Do you have any?