Once Sage Shilad did a tremendous penance to please lord Shiv with an aspiration to have a son. Lord Shiv appeared before him and asked him to demand any boon he wished. Sage Shilad expressed his desire of having a son who is not born from a physical body and who is proficient in all the scriptural knowledge. Lord Shiv blessed him by saying - "So be it". Sage Shilad then returned to his hermitage and performed a Yagya. From the Yagya Kund appeared a child who possessed four arms and three eyes. Sage Shilad was very pleased to see that child. The birth of the child was celebrated with great fanfare. Lord Shiv and Paarvatee arrived to bless the child. The child was named Nandee as his birth had given immense joy (Aanand) to sage Shilad. Later on Nandee lived with his father like any other normal child and was brought up with great love and care. He became proficient in all the scriptures within seven years.

Being inspired by Lord Shiv, two Braahman came to sage Shilad and informed him that after one year Nandee may return back. Sage Shilad became extremely sad. Seeing his father in his sorrowful mood, Nandee consoled him and later on went to do penance. His tremendous penance pleased Lord Shiv and Paarvatee and both of them appeared before him. Lord Shiv blessed him and said- "You are just like me, so you will never die". Lord Shiv also gave one of his garlands to him. As soon as Nandee wore that garland he imbibed all the qualities of Lord Shiv. After that Lord Shiv took out some water from his locks of hair and sprinkled on him, as a result of which five rivers came into existence. These five rivers were later on came to be known as Pancha-nad. Lord Shiv then made him the leader of all his Gan. Later on Paarvatee took Nandi under her guidance and considered him just like her own son. Nandee was married to Suyashaa, the daughter of Marut. Ultimately all of them accompanied Lord Shiv to his abode. There was no effect of the words of Braahman.

Nandikeswar is an incarnation of Lord Shiv himself as told in Shiv Puraan by sage Soot Jee.

Yet Another Story About Nandee
Another is a story about Nandee getting the form of the oxen. This is also from the Internet. One day Lord Shiv called Nandee and asked him to go over to the world and tell people to take oil bath daily and to take food once a week. It appears Nandee was absent minded a little. He came and told the people just the reverse - Take food daily and oil bath once a week. Shiv became angry at Nandee for telling just the reverse of what he has told him to say. So Shiv transformed him into an ox by the curse. It is told because of this all the shortage of food has happened in the world. Just think we all feel hungry only one day in a week. Food production will get lessened, people will work still less, since they require food on one day only.

Another Legend
There is one another story about Vadakkunaath Temple. It is also from Internet.

Once a smart young gentleman, with a very fair complexion, appeared offering prayers sitting in the Mandap. He would appear suddenly, sit calmly offering prayers in the Mandap and would suddenly disappear too. This continued for months. People coming to worship in the temple stood looking at the handsome gentle man including pregnant women. It is said that the children delivered (from those pregnant women who looked at him) started appearing like the same handsome man. And it was not the case only with one pregnant woman but for many pregnant women. Quarrels started on the integrity of women, because they looked at the gentleman. One fine day the gentleman disappeared. It is told that that young man was Lord Shiv himself in the legend. Leaving the legend, generally many people from Trichoor, both male or female, look more handsome and well built and of wheatish complexion comparing to other parts of Kerala.