The section on mounted combat in the Player's Handbook says:
While you're mounted, you have two options. You can either control the mount or allow it to act independently. Intelligent creatures, such as dragons, act independently.
So, any mount can be allowed to act independently. However intelligent creatures always act independently.
However, I cannot find any rules outlining rules or guidelines for what qualifies a creature as intelligent (for this purpose or any other).
As a point of reference, the lowest intelligence score I can find for a dragon (white dragon wyrmling) is 5.
So, how do I determine if a mount is too intelligent to be controllable according to the rules? At what point or under what conditions is a creature considered intelligent?
If there are no rules, I'll accept experience-backed guidance from people who have had to deal with this issue before and had a certain method work or not.