Myfanwy

Myfanwy was a pteranodon kept by Torchwood Three as a pet.

History
Myfanwy was a pteranodon, possibly from an alternate Earth where its kind never became extinct. (TW: Meat) It came through the Rift in 2007. (TW: Everything Changes)

It was originally found hunting in the countryside, killing sheep at night. (TW: Everything Changes) Eventually, Ianto Jones was able to trap it in a warehouse and persuaded Jack Harkness to help him catch it. (TW: Fragments) Ianto named it "Myfanwy" after a Welsh goddess with beautiful singing (ironically, as the pteranodon's screeches sound horrible).

It became Torchwood Three's mascot and pet, and an essential part of their everyday life, even taking part in a basketball game (TW: Cyberwoman)

Jack Harkness was later able to use the pteronodon to attack Lisa Hallett, mortally injuring her. (TW: Cyberwoman) The pteronodon, on the other hand, survived the encounter. (TW: Meat)

It is unknown if Myfanwy survived the destruction of Torchwood Three. (TW: Children of Earth: Day One)

Personality
Myfanwy came and went from the main base as it pleased, entering through a large opening on the ceiling. It was well-trained: it returned to the base on hearing a signal from Jack Harkness's wristwatch, and only ate food treated with a certain "barbecue sauce", which Suzie Costello had developed. (WEB: torchwood.org.uk, TW: Cyberwoman) The pteranodon also had a fondness for dark chocolate, (TW: Fragments) and would also feed on birds which perched around Torchwood Three (TWN: Another Life).

Behind the Scenes
Torchwood Declassified mentions that the pteranodon was nicknamed "Myfanwy" by the cast. This is later shown to be the case in-universe, when the Torchwood site featured an IM conversation between Ianto and Owen about Myfanwy's feeding habits (WEB: torchwood.org.uk).