Underdog is a dog that was used in National Accident Helpline adverts made by Aardman Animations in the UK. He was first introduced in 2010.
In the ads, he'll often talk with an off-screen voice of Brian Blessed who'll talk negatively about something, to which he'll rebuttal with what the service provides with a sarcastic ending quip.
His appearance is very similar to that of Rex from Rex the Runt, and is possibly based off of him.