Sri Dharma Sastha is venerated as Aiyappa in Kerala at Sabarimalai.
The term Aiyappa is of Tamil origin.
It is the combination of words Aiayan and Appan.
Aiyan means venerable while Appan means Father.
The term may mean Venerable Father.
The term Aiyan is used to denote Shiva and Appan Vishnu in Tamil Bhakthi Literature.
Note the Images of Aiyappa one,two and Three.
The third is what is being worshiped now.
The Asana is different.
Readers may contribute on this aspect.
The Saivite revivalist Appar sang about Shasta as the progeny of Shiva and tirumaal(Vishnu) in one of his Tevarams in the 7th century. The child saint tirugnanasambandar in one of his songs praises ayyanar as celibate god, invincible and terrible in warfare, taking his abode alongside bhootaganas of Lord Siva. The place sanctity and history document or sthalapuranam of tiruvanaikkaval, a saivite temple near trichy, which was first documented by sage kasyapa informs us that sasta once served lord sivan at that site and after being blessed with a vision was instructed by lord to take abode in the outer sanctorum. It says that sasta continues to worship lord during the day of tiruvadirai. Adi sankara also has referred to ayyanar in sivanandalahari in one verse . Some ancient hagiographies have accounted that sri sankara was adeivamsam(divine soul portion) of sree sasta(sevugan), the same way as tirugnana sambandar was a divine portion of skanda and sundarar a divine portion of alalasundarar.
We find refernces in the Silappadikaram of the tamil Sangam period where he is worshiped as Chathan.
Puaranaanuru and Akanaanuru also speak of Chathan.
The earliest reference to Aiynar-Shasta is from the Arcot districtin Tamil Nadu. The stones are dated to the 3rd century C.E. They read “Ayanappa; a shrine to Cattan.” This is followed by another inscription in Uraiyur near Tiruchirapalli which is dated to the 4th century C.E
The sangam Poet Cheeththalai Chathanaar was named after Sastha.
This tradition was followed as Aiyanar in Tamil Villages, where even today Ayyanar is the guarding Deity of many a Villages.
The earliest inscription to Shasta was made in 855 C.E. by an Ay King at the Padmanabhapuram Sivan temple. Independent temples to Shasta are known from the 11th century C.E. Prior to that, Shasta veneration took place in the temples of Shiva and Vishnu, the premier gods of the Hindu pantheon. Since late medieval times, the warrior deity Ayyappa’s following has become very popular in the 20th century.
Citation and Reference.