Lord Ayyapan is a highly revered deity in Hinduism. He is especially popular in South India. He is considered to be the son of Shiva and Vishnu who are the two powerfuls Gods in Hinduisn. Shiva and Vishnu are male Gods. Yet Ayyapan is considered their son. As per Hindu scriptures, Vishnu had assumed the form of damsel of exquisite beauty called Mohini to vanquish the evil demons through “her” charms. Shiva got infatuated with the beautiful lady form of Vishnu and their union resulted in the birth of Ayyapan. So Ayyapan is also called as the “son of Harihara” or a fusion deity of Hari and Hara, the names given to Vishnu and Shiva respectively.
There is a famous legend about the origin of Lord Ayyapan. Therere once was a kingdom of Pantalam in India. One day the king of Pantalam found a baby boy in a forest. The king carried the baby to an ascetic in the forest to inquire about the boy. The ascetic advised the king to take the baby home, raise him like his own son, and that in 12 years he would discover who the baby was. The royal family did so, naming the baby Manikantha.
At age 12, the king wanted to formally coronate Manikantha as the heir prince . However, the queen under the influence of an evil minister objected. The minister had advised the queen that only her younger biological child should be the next king. The younger child lacked the ability to perform the duties of the king, something that the scheming evil minister thought would make him the de facto ruler. The minister persuaded the queen to feign an illness, ask for “tiger’s milk” to cure her illness and demand that Manikantha be sent to get the milk from the forest. Manikantha happily volunteered and went into the forest. He returned riding a tigress. The king, realised Manikantha’s special ability and recognized his adopted son to be a divine being, and resolved to make a shrine for him. Manikantha shot an arrow that landed thirty kilometers away. The young boy then transformed into Ayyappan. The place where arrow landed is now an Ayyappa shrine, a site of a major pilgrimage that is particularly popular for visits on holy day of Makara Sankranti (around 14 January, the first day of sun’s transit into the Makara (Capricorn), with the winter solstice the day when Sun crosses).
The above core story is shared wherever Ayyappan is revered in India. There are many temples in Kerala (a Southern state of India) whose presiding deity is Ayyappan, the most famous among them being the Sabarimala temple. The temple attracts millions of visitors every year. Other important temples are Kulathupuzha Sastha Temple, Aryankavu Sastha Temple, Achankovil Sree Dharmasastha Temple, Erumely Sree Dharmasastha Temple and Ponnambalamedu Temple. Ayyappan temples typically show him as a celibate yogi. A few important temples such as the one at Achankovil Sree Dharmasastha Temple near Travancore, however, depict him as a married man with two wives Poorna and Pushkala, as well as a son Satyaka.This unusual temple is believed to have been established by Parashurama, an earlier avatar of Lord Vishnu.
Note: Condensed from Wikipedia Link