Macbeth is attempting to goad the two men into accepting the mission of murdering Banquo and Fleance by asking them to think about the kind of men they are. He makes an analogy by listing types of different dogs:
. . . hounds and greyhounds, mongrels, spaniels, curs,
Shoughs, water-rugs, and demi-wolves are clept
All by the name of dogs
This implies that there are all sorts of men, just as there are all types of dogs. Macbeth
wants the men to feel that they have to prove to him that they are more highly placed in the hierarchy of men just as some dogs are "better" than others. In this way, he hopes to manipulate them into demonstrating to him that they are fierce and loyal, two qualities that henchmen of a king would possess. Not coincidentally for Macbeth's purpose, these would also be desirable qualities in a king's canine companion.