Our dear Echo had a wonderful, crazy Jack Russel friend. The dog was 1/4 Echo's size and weight but could keep up with her, and Echo was fast like a Border Collie. When Echo wasn't playing she was calm like a typical Bernese Mt Dog, not the Jack Russell. This dog rarely stopped.
Since Jack Russell's are notorious for being over active dogs the family searched very carefully for a calm one before adopting their second Jack Russel.