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). (WEB: torchwood.org.uk)

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)

When an Apatosaurus-like alien was accidentally brought into the Torchwood hub, Myfanwy tried to fight it, allowing Toshiko Sato to get to the armoury. After it was destroyed by a Jamolean lance, Myfanwy seemed to take pleasure on feeding on the burnt corpse. (TWN: Pack Animals)

It is unknown if Myfanwy survived the destruction of Torchwood Three. (TW: Children of Earth: Day One) Though it seems unlikely as no one is not going to notice a Pterodactly flying around cardiff.

Personality
Myfanwy came and went from the main base as it pleased, entering through a large opening on the ceiling. Jack Harkness trained it well enough so that it it returned to the base on hearing a signal from his wristwatch, it never attacked a member of staff, never made a mess and only ate food treated with a certain "barbecue sauce", which Suzie Costello had developed. (WEB: torchwood.org.uk, TW: Cyberwoman, TWN: Pack Animals) The pteranodon also had a fondness for dark chocolate, (TW: Fragments) cod, (WEB: torchwood.org.uk) 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)

Jack refers to the beast as a dinosaur but it is not as Pteranodons were pterosaurs, flying reptiles, while dinosaurs only lived on the land