The idol of Guruvayurappan came directly from Vaikuntha. Here goes the story.
Brahma told Vishnu that I can't see you, you are so big, so vast, unfathomable, how do I worship you. That’s when Mahavishnu himself manifested this idol of his form with 4 arms and gave it to Brahma. Later on, Brahma handed over this idol to the Prajapatis, the creators of the world. Finally, this idol reached Vasudeva, the father of Krishna.
Vasudeva was worshipping the idol inside the prison. When Krishna was born, he was born in the same form as the idol. Vasudeva prayed to him to take the normal two-handed human form. That's when Krishna took a normal human form.
Ultimately, the Vishnu idol came to Krishna, and he established a temple in Dwaraka and the idol was being worshiped there. When it was time for Krishna to leave his body, he realised that Dwaraka is going to be submerged under water. Hence, he called Uddhava and asked him to inform Brihaspati to take the idol to another place. By the time Uddhava informed Brihaspati, Dwaraka has already gone into water. Brihaspati then called his disciples Vayu and Varuna and rescued the idol from under the water. Then he explored the option for finding a suitable land to establish this idol. That's when he met Parashurama, who told him about this new land called Kerala and advised him to take the idol there.
When Brihaspati reached Kerala, he saw Shiva and Parvati there and they confirmed the place to consecrate the Idol. Vishvakarma constructed the temple for the idol, and it was finally consecrated there. It was Lord Shiva and Parvati who offered the first puja there. This place was them named as Guruvayur and the idol called Guruvayurappan.
Thus, the essence of the Guruvayurappan idol goes all the way back to Dwaraka. Guruvayurappan is worshipped as Shri Krishna in his child form as an 8-year-old child. The deity represents the purna rupa (full manifestation) of Lord Vishnu, as revealed by baby Krishna to his parents. The idol of the deity is made of a stone called “Patala Anjanam” or black bismuth and is in the standing pose with four arms, carrying the Panchajanya (shank or conch), the Sudarshana Chakra (chakra or disc), the Koumodaki gada or mace) and padma (lotus).