There are many legends, as to why there is a snake around Lord Shiva’s Neck. We will present few of these legends.
As per the Matsya Purana (Hindu Mythology), Vasuki is Shiva’s snake and is also considered as the king serpent known to possess a gem called as Naagmani in its head. The picture of Shiva with a snake around his neck symbolizes controlling of ego which when controlled can be worn as an ornament, in this scenario the snake represents ego.
Vasuki has a sister known by the name of Manasa. In Buddhist mythology (Chinese and Japanese) he is known as one of the eight “Great Dragon Kings”…as per wiki
According to Puranas, the most popular belief is that during Samudra Manthan, Shiva had to drink poison to save the World. It was Vasuki that served as a rope tied to the Mandar Mountain to churn the ocean of milk, and when Shiva drank the poison there were some snakes that drank the poison with him and Vasuki was one of the serpents, impressed by this Shiva accepted Vasuki and termed him as (king of sarpas).
Lord Shiva wearing a deadly snake around his neck signifies that he is independent of time and death and as a matter of fact is in control of Time.
To add to all this, there is one more popular belief as to why Shiva has a snake around his neck.
The story goes like during the churning of the cosmic ocean when Shiva and Vasuki drank the poison, the snake around Shiva’s throat helps to keep the poison in his throat and does not let it slide down into his body.
The three coils of Vasuki around Shiva’s neck signify “Past, Present and Future” stating that Shiva controls time and with Hood of the Serpent on the right signifies that Shiva’s perpetual laws of reason and justice are to preserve the natural order in the Universe.
There is deep symbolism behind the whole Samudra Manthan episode which when laid out in layman’s terms comes to this conclusion:
Snakes are considered evil and demonical, thus by wearing a snake around his neck Shiva assures that no evil can touch or destroy his followers.
The snake also signifies the power of Kundalini which is often described as a coiled serpent that is lying dormant in the Root Chakra (Muladhara Chakra) and ascends upwards when one starts his or her spiritual journey.
The snake around Shiva’s neck conveys a message that kundalini in Shiva has arisen fully and is actively involved in the divine activity thus, by Shiva is keeping an eye on all devotees approaching him with their individual problems.
Since snakes represent anger and are considered poisonous, thus by wearing a snake around his neck and controlling the snake, Lord Shiva teaches us that how important it is for us humans to control anger which resides within us and thus by controlling it we can overcome the poisonous nature of this deadly emotion.