This question already has an answer here:
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.
You can control a mount only if it has been trained to accept a rider. Domesticated horses, donkeys, and similar creatures are assumed to have such training. The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options; Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it.
I own a horse and it is trained to come when I whistle.
At the start of combat I am unmounted and I whistle (not an action). On the horses turn it spends its action to disengage (avoiding OA) and spends all of its movement to come to my side. On my turn (in the same round of combat) I spend half my movement to mount the horse. The horses initiative changes to match mine.
Question: Can the horse spend movement and take an action immediately even if it already spent those resources?
This would allow a mount to have an extra action/movement every time it is mounted. Taken to the ridiculous extreme, it would also allow faster-than-light travel as the horse is mounted, moves, dismounted, and mounted again by another character in succession.
Note that the object of my question is NOT to find out if I can travel through time on my horse given enough help, but just to find out if my horse can re-use its move/actions after being mounted.
Inspired by this question.