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. Employees who tended to the animals were called zookeepers.

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". (PROSE: 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. (COMIC: 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. (COMIC: The Didus Expedition) In 1960s New York City, the Second Doctor stopped a mad scientist from using reanimated dinosaurs to liberate zoo animals. (COMIC: 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 Mathews. Their goal, however, was not freeing the animals, but restoring order to the zoo. (COMIC: The Arkwood Experiments) The Doctor later took the Brigadier's godson Albert to a London zoo, where the young boy made his way into a tiger's cage; the Doctor rescued him, then later helped pay for a larger enclosure for the tiger. (AUDIO: Walls of Confinement)

By his sixth incarnation, the Doctor had a zoo onboard the TARDIS which mostly held endangered species. (COMIC: Changes)

Zoos and companions
Dodo Chaplet once believed the TARDIS had landed in Whipsnade Zoo, when it had landed on the Ark in the far future. (TV: The Ark)

Martha Jones requested to be taken to a zoo when allowed to choose her destination. Unfortunately, the request offended the Tenth Doctor. Nevertheless they did end up going to the Museum of the Last Ones which retained the last examples of several endangered species from Earth. (PROSE: The Last Dodo)

There were no zoos on Traken. Nyssa was shocked at the idea of keeping animals in cages. (AUDIO: Smoke and Mirrors)