Which of the following is NOT a sign of overtraining or overloading in horses?

Increased agility is the correct answer because it does not align with the signs typically associated with overtraining or overloading in horses. Overtraining often manifests as fatigue, reduced performance, and various negative physiological indicators, such as a dull coat, poor appetite, and disinterest in work, all of which suggest that the horse is not functioning optimally.

In contrast, increased agility implies that the horse is responding well to its training regimen, demonstrating improved physical capabilities and readiness for work. This improvement is contradictory to the general decline in performance observed in situations of overtraining. Therefore, if a horse shows increased agility, it indicates that it is likely benefiting from its training rather than suffering from overtraining.