After some practice, here is a V2 of the hairworks tail. 
It's remade entirely from the ground up and now has idle animations. 
Of course, it's still not perfect since it's just impossible to make it perfect, but it's a huge improvement in my humble opinion. 
Things written below are still valid for this new version. 

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So, I learned hairworks, and this is my first asset, made for training. And I wanted to make one tail like that anyway.

Be aware that the speed of the horse doesn't exist because the animations are just loops that doesn't change the position of the horse. So this means that speed has no effect on the hairworks assets when you are on a horse. And you can see that on the mane and on Geralt's hair aleady. 

However I managed to fake a "speed effect" with legs collisions. But it can't really get better than that. 

And now you understand why there wasn't a hairworks tail in the released game. ;) 

I also added a wetness effect to the tail and the mane, an by doing so I fixed a vanilla issue. Now, horse's items, saddles, bags, etc become wet when it rains. That wasn't the case before.