Okay, so they're not actually human. They're still the two most important people—characters? beings? creatures?—in Billy's life.
Being dogs and all, their motivations are pretty straightforward: (1) hunt raccoons, and (2) go wherever Billy goes. This second motivation causes the occasional problem: "There was only one place I didn't want them to go with me and that was Grandpa's store. Other dogs were always there, and it seemed as if they all wanted to jump on Old Dan" (10.8).
While this is a bit of foreshadowing of the fight to come between Old Dan and Old Blue, it also tells us something about Old Dan. While something about him just makes other dogs want to fight, he's sweet and caring with Little Ann. When Grandpa dumps some tasty-smelling canned corned beef in front of him, he'll only lick it until Little Ann gets her share (15). Now that's a true gentleman—er, dog.
Opposites must attract, because these two dogs are as different as they come. Old Dan is bold, strong, and stubborn—we'd say he was the "act first, think later" type, except we're not sure dogs ever really think at all. Anyway, this is the dog who's so stubborn he'd rather freeze to death than let a coon get away.
Little Ann, however, is more of the "look before you leap" type. That's why Little Ann isn't all scarred up the way Old Dan is:
His face and ears were a mass of old scars, caused by the many fights with tough old coons and bobcats. […] I looked Little Ann over and couldn't see any scars. I laughed because I knew why. She was too smart to walk right up in the face of a fight. She would wait until Old Dan took hold and then dart in. (15.76-77)
Got it? They're the odd couple of the animal kingdom. He rushes in; she comes in to clean up the mess.
You might think they'd get irritated with each other—um, if they were capable of feeling irritation, that is—but nope. As much as these two love Billy, they love each other just as much. No, not in a romantic Lady and the Tramp kind of way. Although, they do share food—much to the surprise of Billy's dad, and anyone who's ever seen two dogs eating at the same time:
One of the girls threw two cold biscuits out in the back yard to Old Dan […] picking both of them up in his mouth […]. He walked up in front of the doghouse, laid them down, and growled […] Little Ann came out of the doghouse and each of them ate a biscuit. (15.20)
Sharing food? That's true love. But really, it's more than that. They need each other, and just as one won't go hunting without the other, one can't live without the other either. When Old Dan dies, we expect that Little Ann will die, too.
We're getting a little bummed thinking about these dogs, so let's bow out with one last thought: when you combine Old Dan's stubbornness with Little Ann's smarts, what do you get?