Myfanwy

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

History
Myfanwy was a pteranodon, possibly from an alternative timeline where its kind never became extinct. (TW: Meat) It came through the Cardiff rift to 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, the pteronodon'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).