The earliest memories Atticus has are of living in a forest filled with Bug and Flying Type creatures he could easily fight as a young Houndour. He had plenty of food and no one was in contest with him for water. He doesn't how he ended up there, no memory of a family or other members of his species living there.
Regardless, he was rather content with his territory, but one day a boy ran through the woods carrying a baby Pokemon in his arms. Suspicious of the stranger, and possibly a little curious, Atticus followed him from a distance at which he wouldn't be seen.
The Houndour didn't have any strong feelings to them, but did feel a sense of injustice whenever a Pokemon tried to attack them. Any time a bug or bird tried to, Atticus would jump in to discourage the Pokemon, or fight them off if he had to. Most Pokemon, though, respected the dog. Many of them were already familiar with him, and knew not to put up a fight.
Soon enough, though, the boy had made it to the edge of the forest. The small Houndour, though he did not acknowledge it, was a tinge sad to see him go. So, he approached him and the small Pokemon with him for one last farwell before the Houndour would go back to his regular life.
That was when the boy asked him if he wanted to come with him.
Gold named the Houndour "Atticus" because it sounds like the word "Attack". Silklay couldn't fight at the time, so Gold relied on Atticus entirely for protection. He was the first Pokemon Gold caught, but also his first actually battling Pokemon as well.
The Houndour was very obedient and loyal, as that comes naturally to dog Pokemon. He was able to help train Silklay to learn how to fight, and won Gold most of his early battles
Atticus is INCREDIBLY scary as a Houndoom. He usually snarls instead of yips, growls instead of barks, and will bare his teeth at the slightest provication. This makes people understandably scared of the Houndoom, but actually Atticus is not violent at all and has never attacked a human and never seriously injured a Pokemon, always staying strictly within the rules.
It's normal for Houndoom to act this way, as per their Dark typing. Most people aren't aware of this quality though, for a number of reasons.
Most Houndoom seen in captivity are pets from breeders, and most of them selectively breed away the trait as undesirable. This is especially so when it comes to those who put purebred Houndour and Houndoom for shows. It's hard to find Houndour in the wild at all, and wild ones are difficult to train. When someone does catch a wild one, they usually train the Pokemon out of this behavior. Gold never bothered, as he didn't see it as an issue.