The Jack Russell terrier is a high-energy, extremely prey-driven little dog bred in the 1800s to hunt foxes. Jack Russell terriers may be aggressive toward other dogs if they are not socialized at an early age. Well-trained and socialized Jacks make wonderful, lively family dogs and can coexist peacefully with other dogs. However, some dogs will be able to adapt better than others to life with a typical Jack Russell terrier. Not all dogs behave according to the standard expectations for their breeds, so it's important to observe how your own dogs behave rather than assuming two dogs will get along because of the normal characteristics of their breeds.
Most terriers tend to have high energy levels, so another terrier may be able to match your Jack Russell terrier's energy level without becoming annoyed. Compatible terriers may include cairn terriers, West Highland white terriers, miniature schnauzers, smooth fox terriers and mixes of these breeds. Jack Russells often pester more sedentary dogs for attention until the harassed dog snaps or growls, but this problem may be avoided with a dog of similar size, age and energy level.
Herding dogs such as border collies and German shepherd dogs may match the Jack Russell's renowned energy. These dogs are also large enough to dominate a Jack Russell, preventing the smaller dog from running the house. However, some herding dogs will chase and nip at small animals, including small dogs. A herding dog who has not been socialized with smaller dogs may not make a good companion for Jack Russell terriers.
Lap dogs such as the Chihuahua, Yorkshire terrier, bichon frise or Shih Tzu are small, highly social dogs that are similar in size and intelligence to the Jack Russell terrier. When the Jack Russell and the lap dog grow up together, they can make excellent companions. However, Jack Russell terriers and lap dogs tend to be very demanding of attention, and jealousy can occur. Many lap dogs are quick to bite out of fear, and some Jack Russells also may be fear biters, so the arrangement only works when both dogs are well-socialized.
Every dog is an individual, and socialization and environment can radically alter a dog's disposition. As a general rule, however, dogs who tend to be dog-aggressive are poor choices as companions for Jack Russell terriers. This is because the Jack Russells can be highly persistent and territorial, and may provoke aggression in a dog already predisposed toward such behavior. If the other dog is very large, this can be especially dangerous for the little Jack Russell. Among breeds predisposed to dog aggression are the American Staffordshire terrier or American pit bull terrier, presa canarios and chow chows. Any dog who has shown signs of aggression toward other dogs should not be left unsupervised with a Jack Russell.