Actually this is partly because since people feared vicious dogs, it was only natural for them to compare them jestingly with the even more ferocious tiger, as in this joke about an "illiterate dog": In olden times legend had it that the written character for tiger, or the character for the tiger's associated Earthly Branch yin, had the power to subdue a dog.
其實這也是因為人怕惡犬，不免藉比狗更勇猛的老虎，來調侃它，例如一個「狗文盲」的笑話：