If you were dreaming of a snail, it could be an inference to somethings in your waking life. If you had been gardening and seen snails, then your dream was triggered by this event. Snails are harmful to plants as they eat their leaves.
If you think you know someone who behaves like a slug in your waking life, then this dream is a reminder to you, to take care of that snail. You should prepare counter measures to act against the destruction, or prevent destruction from happening.
Prompt from R. Shipp – snail.