"Dog" has a couple of meanings. One of the more common of the newer meanings is the same as "player". It's originally from the jazz world, when a "player" was sometimes called a "bird dog".
There was always a charmer in the band who showed up for the gig with a pretty girl on his arm, no matter where the tour took them. Like a good bird dog could always find the bird in even the thickest undergrowth, a "bird dog" could always find the best of the local "birds", even in the unlikeliest of small towns.
Or else it just means "member of the pack"... a buddy, a pal, a friend. Both usages are pretty common.