Serpent

The serpent, or snake, is one of the oldest and most widespread form of Fertility symbolism. In terms of Creationism, the Abrahamic religions, Judaism, Christianity, and Islam, narrowed the view of the the serpent to sexual desire. However, in Hinduism, the symbol of the serpent is broadened where Kundalini energy is coiled by two serpents, representing the residual power of pure desire*.

Penis symbolism
The serpent was also recognized as a phallic symbol and subsequently associated with water. The Hebrew Bible uses such explicit phallic imagery in association with the serpent, as in the following dialogue between Moses and the LORD:
 * The LORD said to him, “What is that in your hand?” And he said, “A staff.”
 * Then He said, “Throw it on the ground.” So he threw it on the ground, and it became a serpent; and Moses fled from it.
 * But the LORD said to Moses, “Stretch out your hand and grasp it by its tail”—so he stretched out his hand and caught it, and it became a staff in his hand. (Exodus 4:1-5)

Numbers 20:7-11 is also considered to be easily conceived as phallic when Moses uses his rod to bring forth water.

The Hopi people even used rattlesnakes for use in their fertility dances as a sacred means to bring the rain.

Vagina symbolism
Because opposites are often evoked in body symbolism, it is not surprising to find traditions that link the serpent to the vagina as well as to the penis. There is the folk belief that menses is caused by a serpent bite that produces a magic wound that periodically bleeds but is never healed.

Folklore Hindu Kali is a mother whose vagina dentate (fanged vagina) devours the male penis. This concept is reminiscent of The Ouroboros, an ancient symbol of a snake, serpent, or dragon eating its own tail, denoting self-cannibalism.

The Ouroboros often symbolizes self-reflexivity or cyclicality, especially in the sense of something constantly re-creating itself, the eternal return, and other things such as the phoenix which operate in cycles that begin anew as soon as they end. It can also represent the idea of primordial unity related to something existing in or persisting from the beginning with such force or qualities it cannot be extinguished. While first emerging in Ancient Egypt, the Ouroboros has been important in religious and mythological symbolism, but has also been frequently used in alchemical illustrations, where it symbolizes the circular nature of the alchemist's opus. It is also often associated with Gnosticism, and Hermeticism.

Carl Jung interpreted the Ouroboros as having an archetypal significance to the human psyche. The Jungian psychologist Erich Neumann writes of it as a representation of the pre-ego "dawn state", depicting the undifferentiated infancy experience of both mankind and the individual child.