A monster expert was a person who demonstrated an expertise in monsters. During USS Voyager's encounter with a bioplasmic telepathic pitcher plant in 2375, the crew was assisted by Qatai, an alien who had been hunting the "beast" for nearly forty years.
After Qatai explained the crew was being telepathically manipulated into deactivating The Doctor, who had recently been reactivated and not yet introduced to Qatai sarcastically inquired, "And who might you be, the local monster expert?" Qatai concurred, stating that "as a matter of fact I am." before he further explained the nature of the organism. (VOY: "Bliss")