You should probably unlock this naturally by progressing through the game, but if not its easy to get. Dogs can have accessories in various slots (outfit, hat, glasses, collar, shoes). Dogs must be wearing an accessory in at least two of these slots to count toward this. LIKE THE SLEEPING DOG ACHIEVEMENT, THIS IS FOR 20 DIFFERENT DOGS, TAKING 20 OF ONE WILL NOT WORK. Fairly early on you unlock the ability to dress any dogs, which makes this achievement a piece of cake if you haven't unlocked it yet.