Final answer:
The insurer's mandate is likely due to a per-incident limit, which is a maximum amount the insurance company will pay for each separate illness or injury.
Step-by-step explanation:
The most likely explanation for the insurer's mandate of not paying for additional care of this condition is a per-incident limit. A per-incident limit is a maximum amount that insurance companies will pay for each separate illness or injury. In this case, the removal of the mandibular mass was considered a single incident, and the insurer may have reached the limit for coverage related to that incident. Therefore, the client would need to pay for any follow-up care for the horse's condition.