Jack Russell Terrier? My Jack is 11 years old....we got him right before we got married, so he was our very first baby. He's great with cats, kids, and other animals. In fact, we used to have degus (sort of like gerbils) and if they escaped, I'd set my Jack loose in the house searching for them. He'd locate them without harming them. He does love to chase after squirrels and cats if they enter the yard....but not to harm them. He just wants to play.
He's energetic when I'm playing with him or someone enters the house, but for the rest of the day, he's really lax and just finds a place to sleep.