[RSQRottweiler]

My almost-3 year old Rottweiler (neutered male), Sebastian, gets along great with other dogs. I take him to work with me, where my boss has a neutered male Lab -- they pretty much ignore each other, but play occasionally. At the farm where I board my horse, there is a small Lab mix and an even-smaller TFT mix. They all get along beautifully. When I go to visit my parents, they have a female Rottweiler (spayed) named Gretchen, and a couple of the neighbors have Labs (one black, one chocolate - both spayed females). It's not uncommon for us to stop and talk, and the dogs just lie in the grass without any aggression towards each other, and without having to be in each other's face. In obedience classes, the trainer's Border Collies are always used as "distractions" or obstacles during training -- never a problem with that. Various other experience with dogs of all sizes, shapes, colors, breeds/mixes, and training levels (including one RottStock in 2001). No problems there, either.

Now here's the problem: there is a Pembroke Welsh Corgi in my parents' neighborhood that, for whatever reason, Sebastian hates. I have no idea why. They have never "met" and the Corgi is very quiet and well-behaved. However, on Sunday night I was visiting my parents and my mother and I had the dogs out for a walk when we stopped to talk to the two Lab owners. All the dogs were flopped-out on the grass (still on-leash). A couple of rollerbladers went by, right behind Sebastian, and he didn't even move.

THEN the lady with the Cordi came by, and Sebastian went off -- barking, lunging, growling -- like I've never seen him before. The Corgi did nothing to provoke a fight -- dog and owner were on the sidewalk on the opposite side of the street. I took Sebastian out of the way and did several minutes of heeling exercises, partially to calm him down, and partially to calm myself down. After that, he returned to normal and relaxed with the other dogs. One of the Labs even crawled over and licked his face, and Sebastian just wagged his tail.

What could have set my dog off like this? He's never done that to any other dog but that Corgi. I don't think it's a size issue, because he's been around dogs smaller than the Corgi without any problems. I don't think it's a gender issue, because he's been around intact and neutered males and females.

And ideas?