Zoo

A zoo was an institution which housed animals so that other animals 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 them 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 did free 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 quite unclear that 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) 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 merely returning the normal order to the zoo. (TVC: The Arkwood Experiments) In his his sixth incarnation had a zoo onboard the TARDIS which mostly kept endangered species. (DWM: Changes)

Zoos and companions
Although no companion was known to be a zookeeper, some companions did occasionally remark upon them or express a desire to visit them. Dodo Chaplet, for instance, once believed the TARDIS had landed in Whipsnade Zoo, when in fact it had landed on the Ark. (DW: The Ark) Martha Jones had 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)