Dog is Alyx's robot bodyguard and playmate. Dog was first found in the Black Mesa East chapter. Alyx Vance's dad made Dog to pertect Alyx against the aliens. Dog is a strong fighter, you will see him throw a van at a group of Combine Soldiers and destroy a Combine APC at a building, even pull of a brian out of a Strider! Dog also saved Alyx against a Combine Advisor.
Dog has a ball that looks like a Rollermine and a face from a destroyed Scanner Type 1, with 3 plates instead of 4 and repainted.
1. Any four-legged, pack hunting, carnivorous animal of the genus Canis. Yes, I consider a wolf to be a dog, because the common house Dog, Canis Familiaris or Canis Domesticus is a bred-down version of Canis Lupus, the modern wolf. Makes a good pet, as they are intelligent, curious, and as I have earlier stated, a pack animal, and therefore easily suited to life with a family of many people, or fewer.
2. What you call a close buddy.
1. Peta can suck my nuts, dogs, and every other pet, usually (90% of the time,) have a better life in captivity than in the wild, as a matter of fact, if it weren't for human captivity, we wouldn't have Canis Familiaris as we know it today! LICK MY ASSHOLE, PETA!