Dog Becomes Watchdog

Listen up, buttercup. The joke is playing on the words "watchdog" and "watch". A watchdog is a dog that barks at strangers and a watch is something you wear on your wrist to tell time. By giving your dog a Rolex (a fancy watch), you turn him into a "watchdog". You probably need to get the joke explained since your brain is still catching up from dial-up internet.