Guruvayurappan, is a from of Vishnu worshipped mainly in Kerala. He is the presiding deity of Guruvayoor temple. Who is being worshipped as Shri Krishna in his child form (Balagopalan), popularly known as Guruvayoor Unnikrishan (Guruvayur Baby Krishna ) Even though the deity is that of Chatur Bahu (four handed) Vishnu, the concept (Sankalpam) of the people is that the deity is the infant form of Lord Krishna.
Origin of Guruvayoorappan
King Sutapa and his wife Prishni prayed to Brahma for a child. Brahma, the consent of Vishnu, gave the king a murti of Krishna given to him by Vishnu himself. It is said that by the grace of this murti. Brahma was able to fulfill his task of creation.
When Vishnu appeared before the king and the queen in answer to their prayers, they prayed to the Lord for a son like him. As they repeated it thrice, the Lord told them that they would have three janmas(birth) and that he would be born to them in each their three incarnations. In due course the Lord was born to them. He was born to them at first time as Prisni Garbha, who taught people the importance of Brahmacharya. The second time he was born to Kashyapa and Aditi and took form as the avatara of Vamana. In their third incarnation, they were Vasudeva and Devaki, parents of Krishna. The legend states that Krishna got the murti from his father and worshipped it at his capital, Dwaraga.
The holy idol or murti is almost 4 feet tall and made of a stone called "Patala Anjanam" or black Bismuth and is in the standing pose with four arms, carrying the panchajanya (Shangu or Conch), the Sudarshana chakra ( Chakra or disc), the kowmodagi (Gada or mace), and padma (Lotus).
It is believed that the deity of Guruvayurappan represents the purna rupa (full manifestation) of four armed Vishnu revealed by baby Krishna to his parents Devaki and Vasudeva, immediately after his advent in Kamsa's jail. This idol was, therefore worshipped by the parents of Krishna and later was worshipped by Krishna an avatar of Vishnu himself. So baby Krishna is worshipped on a Vishnu deity.