The dog is a Jack Russell Terrier, which has its origins in fox hunting. The name "Jack Russell" has been used to describe a wide array of small white terriers, but is now most commonly used to describe a working terrier.
It is the same breed of dog that plays Jim Carrey's pet on the Mask.
The red fox is the traditional quarry of the Jack Russell Terrier, and the quarry pursued by the Reverend John Russell himself. Red fox may den in a wide variety of locations from old badger settes and drain pipes to building crawl spaces, old rabbit holes and groundhog dens, but in all cases the working Jack Russell must be small enough to get up to its quarry, which is to say a Jack Russell's chest should be no larger than that of the animal it is pursuing.
Jack Russell Terriers are very intelligent, high-energy dogs – requirements of a working dog which must problem-solve in the field and work tirelessly against often formidable quarry. Due to their compact size, friendly and inquisitive nature, and intelligence, Jack Russells are popular as pets. Prospective buyers should be aware, however, that while these dogs may enjoy sitting in a lap, they are not “lap dogs” – they are dogs that require training and regular and consistent exercise to maintain their temperament and to occupy their minds.
Jack Russell Terriers are very playful.Jack Russells that are not trained on a consistent basis, or are not exercised regularly, may exhibit unmanageable behaviour, including excessive barking, escaping from the yard, or digging in unwanted places inside and outside the house. In America, several Jack Russell rescue networks have to work constantly to find temporary and permanent homes for Jack Russell Terriers whose owners could not meet these requirements for keeping these dogs as house pets. Prospective Jack Russell Terrier owners are advised to do their homework.
Most Jack Russell Terriers easily mingle with children, though they do not tolerate even unintentional abuse. Most are outgoing, and very friendly towards other dogs, but a good number show same-sex aggression issues. Some JRT's exhibit a Napoleon complex regarding larger canines that can get them into dangerous situations. Their fearlessness can scare off a larger animal, but their apparent unawareness of their small size can lead to a lopsided fight with larger dogs if not kept in check.
It is not uncommon for a Jack Russell terrier to be cat-aggressive, and homes with other small fur-bearing animals in them (pet hamsters, rabbits, guinea pigs, ferrets etc.) would do well to think through the ramifications of bringing a working terrier into the house.