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.
Best Answer
Firstly that's not what it means. All mounts can be allowed to act independently. The dumbest horse in the universe doesn't need someone to tell it what to do, and can act independently, without needing orders to walk around, eat, etc. All mounts can act independently.
However, “[i]ntelligent creatures […] act independently” — always. You can't control a dragon mount, you can only coordinate with this intelligent ally who happens to be carrying you. Intelligent creatures can't be controlled like a trained horse, they are asked nicely, bargained with, threatened, or otherwise encouraged to serve you as a mount.
How smart is an “intelligent creature”?
There's no strict rule for this — but when you understand why intelligent mounts can't be controlled and always act independently, it's easy to make such a determination. If it's smart enough to be the rider's ally rather than be a domesticated animal, it's an intelligent creature.