Final answer:
Pari-mutuel wagering is most involved in the horse racing sector of the equine industry, serving as the primary betting system at racetracks and being regulated by law for fairness.
Step-by-step explanation:
The part of the equine industry that most involves pari-mutuel wagering is the horse racing sector. Pari-mutuel wagering is a betting system used in horse racing where bets are pooled together and the payouts are determined by sharing the pool among all winning bets after taxes and the house take are removed. It is an essential aspect of the horse racing industry, as it is the primary form of betting at racetracks and many other horse racing events around the world. This betting system is regulated by law in many jurisdictions to ensure fair play.