Gaius gave Merlin a cat for his 20th birthday. A fat ginger thing with an air of smugness, yet stupidity about it.

Watching it from afar, Merlin soon discovered that his ginger pet could best any other cat in the art of chasing and killing mice.

Problem is it could also best them in the art of getting itself stuck in Gaius's pots and walking into things while it tried to look important.

Merlin decided that his cat was born to kill. Merlin also decided that his cat was a bit of an arrogant Prat.

Merlin called his cat Arthur.