What does he acts like a mad dog mean?

Answer 1

In the past, people refered to "a mad dog" when the dog had rabies. Untreated, rabies affects the body and brain of a dog, so the dog is easily irritated, easily provoked, is unpredictable, and often attacks people (even an owner). Over time, in relation to dogs, it became an analogy used to refer to any visciousness, even without any disease, for example, growling and ripping meat when eating.

When we apply that analogy to a human being, it refers to behaviors, not any illness. For examples:

"He ripped into the T-bone steak like a mad dog!" "He acts like a mad dog when he doesn't get his way."