Zoo

A zoo was an institution which housed animals so that people could look at them. Sometimes, zoos were used less for entertainment and more for the protection of endangered species.

Zoos and the Doctor
The Tenth Doctor once claimed to be physically repelled by zoos because, as he told Martha Jones, "the thought of anything being caged hurts me". (NSA: The Last Dodo) However, on another occasion, he voluntarily visited a zoo on his own. Although he freed a few of the animals, he did so not on moral grounds, but only because it helped him avoid Daleks. (DWBIT: Carnage Zoo - Extermination of the Daleks)

Indeed, it was unclear if other incarnations of the Doctor were particularly bothered by zoos. His first self was quite happy to help a zookeeper recapture his prize didus. (TVC: The Didus Expedition) In 1960s New York City, the Second Doctor stopped a mad scientist from using reanimated dinosaurs to liberate zoo animals. (TVC: The Monsters from the Past) The Third Doctor and the Brigadier once encountered animals in a London zoo whose dispositions had been chemically altered by Cedric Matthews. Their goal, however, was not freeing the animals, but restoring order to the zoo. (TVC: The Arkwood Experiments) In his sixth incarnation. the Doctor had a zoo onboard the TARDIS which mostly held endangered species. (DWM: Changes)

Zoos and companions
Although no companion was known to be a zookeeper, some companions did occasionally remark upon zoos or express a desire to visit them. Dodo Chaplet, for instance, once believed the TARDIS had landed in Whipsnade Zoo, when it had landed on the Ark. (DW: The Ark) Martha Jones once specifically requested to be taken to a zoo when allowed to choose her destination. Unfortunately, the request offended the Doctor. Nevertheless they did end up going to a grand zoo which retained the last examples of several endangered species from Earth. (NSA: The Last Dodo)