Labrador Chasing Smaller Animals

Question
Help! I have a 20 month old male yellow lab.  We got him at 10 weeks old and since then he has grown up around 2 other labs and a pug and a rabbit.  He is walked early in the morning and plays in our fenced yard for hours.  He's always barked at cats and chased them out of the yard however, today he was in the front yard with me on a tie out while I was working in the yard and a pomeranian came into our yard.  He was able to break the tie out chain and made it to the pomeranian and started growling and barking at it.  He didn't harm the other dog but how can I stop this behavior when he's never acted this way towards other small animals he's around like the pug and the rabbit?

Answer
It's pretty normal for a dog to recognize "family members" but see other small quick moving creatures as "prey" Really fortunate that he did not injure the little pom, but of course you can't count on that sort of luck again. It's also normal to want to drive strange animals off of "his" territory, and being tethered can increase the frustration and intensity of that behavior. I'd start introducing him to dogs he doesn't know in a controlled manner like a good group obedience class, and recognize that if he's out front, he needs to be with you and under your direct control. Sandy Case MEd CPDT www.positivelycanine.com