The shadow does not belong in a list of dream symbols in the way that water or a snake does, and the difference is worth making explicit.
Water, snakes and teeth are things people dream about and that various traditions have tried to read. The shadow is not an image people report dreaming; it is a theoretical construct proposed by Carl Jung to explain a class of dream figures. When someone says they dreamt of a shadow figure and looks up the meaning, they are being handed a piece of one psychologist's model as though it were a traditional interpretation, and no older source offers anything comparable.
In Jung's account the shadow is the sum of the qualities, impulses and capacities a person has refused to recognise as their own. It is not the same as evil: it contains whatever the conscious personality has declined to include, which for a conventionally virtuous person may be aggression or appetite, and for someone who identifies with their own darkness may be tenderness. It is formed by the same process that forms the persona, the face turned outward, and the two are correlative.
He held that it appears in dreams as a figure of the same sex as the dreamer, often dark, hostile, pursuing, or contemptible, and that the encounter is the first substantial task in the process he called individuation. He distinguished this from the anima and animus, contrasexual figures with a different function, and from the Self.
Every element of that is a proposal, published under a name, within a system. It has no attested history before the twentieth century.