A mugato "watcher"
A voyeur, also known colloquially as a "peeping Tom" or "watcher", was an individual who derived pleasure from watching, sometimes secretly, others engaged in vulnerable activities.
According to Doctor Zephram Cochrane, he loved a good peep show. (Star Trek: First Contact)
After The Doctor admitted to Captain Kathryn Janeway in 2372 that he was eavesdropping on the bridge, he explained that he was "simply trying to monitor issues involving the health of the crew, Captain. When activated, the Emergency Medical program establishes com-links with all key areas of the ship, and I have, on occasion," before stating, "I'm a doctor, not a voyeur. I am programmed to be discreet." Janeway then referred the situation to Commander Chakotay, who would, "at the earliest opportunity", discuss The Doctor's operating protocols, before ordering "no more eavesdropping." (VOY: "Parturition")
While consulting B'Elanna Torres after he had lost his mobile emitter in a transporter accident in 2375, The Doctor appeared on various monitors in her quarters, including the one in her bathroom. When he appeared there, as she was preparing for a sonic shower, she quickly grabbed a towel to cover up, and, before yelling "hey!". The Doctor stopped what he was staying to quickly remind her that "I'm a doctor, not a peeping Tom. It's nothing I haven't seen before," before continuing his talk. (VOY: "Drone")
Tom Paris alluded to this type of behavior conducted by The Doctor when he recreated the holoprogram Photons Be Free, where he referred to the fictional ship that the story was based on as the USS Voyeur. (VOY: "Author, Author")
While hiding inside a log to avoid two mating mugatos, Sam Rutherford and Brad Boimler discovered that a third mugato, another male, had joined their company. At first, Boimler believed that the situation "could get violent if he tries to assert dominance," before he realized that the second male was there simply because he liked to watch. After the mugatos had worn themselves out with their sexual activities, Rutherford and Boimler emerged from their hiding place and a scene best described as "Ew, gross," as they had found that "even the watcher" was worn out too. (LD: "Mugato, Gumato")