Lord Shiva and Mother Parvati themselves are Lord Hanuman. There is a very interesting story behind this as well.
Story of Hanuman Tail
One legend says that Goddess Parvati tells Lord Shiva that they, too, should have a beautiful palace in which to reside.
To fulfil the Goddess’s wish, Lord Shankar summoned Vishwakarma. Shiva ordered him to build such a magnificent palace that it would put even the palaces of Indra and Kubera to shame.
When Vishwakarma completed the palace, the time came for the Griha Pravesh (house-warming ceremony). For this ceremony, the great scholar and sage Rishi Vishrava (Ravana’s father—some versions of the story also mention Ravana) was invited.
He performed the Griha Pravesh according to all proper rituals. When the time came to give dakshina (the ritual offering), Rishi Vishrava expressed his wish and asked for the palace itself.
Lord Shiva said “Tathastu” (so be it). At this, Goddess Parvati became displeased and resolved that she herself would destroy the palace. However, out of respect for the sanctity of a Brahmin, she controlled her anger.
Later, when Lord Shiva took the incarnation of Hanuman to serve Lord Rama, Goddess Parvati became Hanuman’s tail. Through this, she burned and destroyed the palace along with the entire kingdom of Lanka, thus fulfilling her resolve.
Service of Lord Rama
Another story says, All the gods, along with the Earth, Brahma and Shiva, praised Lord Vishnu. Then He appeared and said, “I will take a human incarnation (Rama) and liberate the Earth from demons.” After that, Lord Shiva also resolved to take a vanara (monkey) incarnation in order to serve Lord Rama.
Mother Parvati said, “I too will go to serve you and the Lord.”
Shiva said, “O Goddess! That will be my brahmachari (celibate) form. Since the Lord will become human, I will become a humble vanara and serve him. He will be famed by the name Rama, and I will be known as Ramaduta Hanuman.”
Upon hearing this, the Goddess decided to become Hanuman Ji’s tail so she could serve both Lord Shiva and Lord Rama.