A dream of a monster like the undead zombie, reflects a recurrent theme in your life. For example, if you face an enemy in your daytime interactions, this theme may surface as an undead zombie, or monster, in your dreams. The threat in your waking reality, can be represented by something hideous in a dream symbol.
A zombie is associated with a former human who does not die, just like an old enemy who does not go away, but resurfaces to stalk you. What you face in reality, is represented in dreams, by a symbol that holds associated meaning with your theme.
If you had watched a TV show before bedtime with a horror theme, your brain has registered horror as some of the last few images you’ve seen before sleep. The impression retained may be strong enough to trigger a nightmare.