Snakes
Freud read the snake as sexual; Jung read it as transformation and the unconscious. In practice the emotional tone of the dream decides — a frightening snake and a calm one are not the same symbol.
Dreams about animals
Spiders
The web matters as much as the spider: a situation you feel caught in, or one you are patiently constructing.
Cats
Often read as the independent or feminine aspect of the self. An aggressive cat suggests intuition being ignored.
Dogs
A friendly dog is usually read as loyalty in your life; an aggressive one as loyalty betrayed or a boundary being guarded too hard.