What I expected to happen was that I can not open the Inventory of the horse when it is not tamed.

What actually happened was I ride a not tamed horse and open the Inventory. Then i can put a Saddle on the Horse an it is tamed. Now I can ride the Horse

1. Find a horse
2. rightclick the horse an ride them
3. Open the Inventory of the horse with your Inventory Key
4. Put the Saddle on the Horse
5. Then close the Inventory
6. The horse have no a saddle and is tamed.
7. Now you can ride the horse normaly
8. By taking the saddle of the horse it is not be tamed anymore

Here is a Video: Click me