Is an employee liable to pay for an injury if he or she was horseplaying and caused an injury or is it covered by workmens comp?

The employee who instigated the "horse-play" will probably be held personally responsible for any injury or damage claim. WCI only applies to injuries incurred when the employee is performing regular job related duties. The injured person can file a lawsuit to recover medical and any other expenses caused by the person who started the rough-housing.