A zookeeper was a person who fed and tended animals in a zoo.
During their initial contact with the Companion, it was initially believed that Zefram Cochrane, and the Galileo crew were captive. After it was discovered that the Companion possessed a feminine persona, Spock stated then it is not a zookeeper." James T. Kirk agreed, and suggested that it was "a lover," and followed by telling the Companion that "it is wrong to keep us here against our will." (TOS: "Metamorphosis")
When The Doctor was mimicked by Leonard, Doctor Lewis Zimmerman's holographic iguana, The Doctor had him deactivated. Insulted, Dr. Zimmerman told The Doctor "how dare you," to which The Doctor replied "I'm a doctor, not a zookeeper." (VOY: "Life Line")