My Jack Russell Story:
We have a 5 year old JRT, named "Fred." He was the runt of his litter, but that makes him all the better. We lived in Colorado when we got him, and lived right up against the mountains. We had an unfenced yard, that deer loved to graze in. Our house was on a hill overlooking the yard, so you could see the whole thing from the back door.
When we got Fred, we had no idea what we were getting into. The second day we had him, we were out on the back porch just messing around with this 3 month old dog. Then the group of about 10 deer came into the yard on their way somewhere. I guess we weren't holding onto Fred hard enough, or we were looking at the deer more than him, but he took off like a Ferrari after these things. He chased them up the mountain that they came from, and we had a fairly good view of it. So we just stood on the porch and watched this little white dot get smaller and smaller chasing these things that were about 20 times his own weight. My dad said that it was good we had a collar with his name on it, because the next day we'd get a call from Wyoming from somebody wanting us to come get our dog. I think that our whole family resigned themselves to the fact that we'd never see Fred again.
Well, about an hour later, he was at our back door whimpering that he wanted in. He came in, lied down and started licking his feet. His pads were full of those little red cactus prickers, but he wouldn't let us touch them. He probably licked for the next week, but eventually got back to being able to walk further than our driveway to go to the bathroom.
He got out a few times after that, and we finally got an invisible fence for him. Until we moved from that house two years ago, he continued to chase deer out of our yard, but he didn't go past the white flags, too many times.