Holy Cow! It's Nandi!

Nandi, as the Bull, driven by Ganesh to carry Shiva and Gauri.
Gauri thoughtfully shares a snack with her son, the driver, Ganesh,
who, because he has four hands, does not even need to put down the reins
or goad to enjoy the snack.
Nandi is the vehicle of Shiva, and also the vehicle of Gauri.  As well as the vehicle of the Matrikas, the nurse mothers of Shiva and Parvati's son Kartikeya.

But more, Nandi is Shiva.

It is probably best to start with what a "vehicle" is: a "vehicle" (or Vahana) is that which "carries" or "transports." In this sense, a vehicle is the means by which the being or existence of Shiva manifests. A "being" is devoid of "force," "power" or "identity." It is the "presence" which the vehicle carries: much as a truck, or a horse, acting as a vehicle, might carry or transport your presence. A vehicle is how you navigate space - and this is exactly what Shiva uses Nandi for.

Shiva, like Vishnu, or Brahma, etc., are not adequately described in terms of space, or time. Or existence. But when existent, Shiva bears force, power, identity, and occupies and traverses space and time. While other vehicles are distinct from that which they are carrying and transporting (Ganesh's vehicle is a mouse, for example), it is not so unusual to use yourself as a vehicle: many people, and most animals, walk there or here on their own feet.

Shiva does not have feet, per-se. But Shiva does have Nandi.

In the Linga Purana 17.3.4, Shilad greatly pleased Shiva by his devotion (which lasted for thousands of years). Shiva wanted to reciprocate the devotion to his devotee, and asked Shilad what he most desired? Shilad said, a self-born and immortal son. Shiva knew what Shilad was getting at: Shilad's devotion to Shiva was like that of a parent for a child - a practice very much like Vatsalya Bhava. Shiva acknowledged Brahma had been trying to convince Shiva to take form again. "I will take birth as your son, and my name will be Nandi. And I shall give you immortality."

In the Kurma Purana 12.33, Shilada was plowing, an act of Yajna Kunda, the land when suddenly a handsome boy appeared on top of the plow! The boy could speak, and called Shilada "Father." The boy, Nandi, studied the shastras and became very learned. Nandi wished nothing more than to see Shiva. And to become immortal, like his father - whom he loved very much. Sitting on the shores of the ocean, he called out to Shiva thousands of times. At last, Shiva appeared (remember that Shiva exists beyond space and time, and so appearing to a form Shiva took is not necessarily more complicated than when a person talks to themselves out loud). Nandi was delighted! Shiva asked Nandi what Nandi wanted? Nandi said, "grant me enough life to call your name out thousands of times more, and see you again!" This was granted. And again and again, Nandi devoted himself to Shiva. But Shiva knew what Nandi actually wanted. The third time, Shiva appeared with Parvati and said "Enough is enough - there is no need to keep calling out My name to see Me. I will make you the guardian of my gates, you shall be my vehicle, you shall be Lord of all the Ganas. You shall be my constant companion!" Shiva then arranged a marriage between Nandi and Suyasha.

But why was Nandi so concerned with death? The Shatrudra Samhita 5.3 describes the Yajna Kunda in which Nandi appeared resulted in tremendous celebration. Shiva and Parvati both arrived to bless the child (again, as both Shiva and Parvati do not exist within time or space, this is no great obstacle to understanding). Visitors kept coming, though - to celebrate. Even seven years later! (People traveled slowly back then). At last, one day, to Brahmans came and informed Shilad that after one year, Nandi would cease to be. Shilad understood this to be "dead" and grew very upset, very sad. After all - he loved his son, and Shiva had promised an immortal child? Seeing his father so upset, Nandi quickly learned what was said. Nandi wished nothing more than to see Shiva - but now also wanted to become immortal, and fulfill his father's faith and desire. When Shiva had told Nandi he would be Shiva's constant companion, and never die, Shiva said "you are just like me, so you will never die." Just because something or someone ceases to be does not mean they die!

Shiva gave to Nandi one of his garlands, and as soon as Nandi put on that garland, Nandi imbibed all the qualities of Shiva. Which is fortunate, because at the "beginning" of things when the poison of Karma spilt across the universe, Nandi was there, beyond space and time, by Shiva's side to drink the poison that dribbled from Shiva's mouth. Like Shiva, Nandi was not killed by the poison, either. And Nandi was guarding Shiva's gates when Gauri asked him to keep everyone out while Gauri bathed - and because Nandi permitted Shiva to enter despite Gauri's order (Nandi could not refuse Shiva anything, since Nandi WAS Shiva), Gauri eventually made Ganesh. Ganesh is known as a Lord of the Ganas, too. But Nandi is the first Gana, and out of penance for disobeying Gauri in preference for Shiva, gives fresh grass to Ganesh annually as a gift (grass is something both Nandi - who is a "bull" - and Ganesh - who is an "elephant" - enjoy). In other stories, Nandi teaches great Gurus - and has many adventures of his own.

You can usually see a picture or statue of Nandi guarding the gates of Shiva's shrines. These usually are seated, facing the main shrine.  Nandi is a master of joy, happiness, and kama.  He is a master of music and dance.

It is interesting to realize, however, that Shiva is both consort and vehicle to his "wife."  This indicates in a special way the unique relationship that develops through love.  And that special relationship, that special love, is extended to the nurse mothers of his child, too.