Apollo, a well-known Greek God, is also a member of the Di Consentes (the Twelve Great Gods) of the Romans. For the Romans, Apollo is first a God of Healing. When Augustus became Emperor in 27 BCE, he promoted the worship of Apollo further. Augustus claimed that he was conceived by this God. After that, Apollo’s Roman attributes expanded to include his Greek ones, including God of the Sun. (Sol Indiges is the traditional God of the Sun for the Romans.)
When a devastating plague hit Rome in the Fifth Century BCE, the Duumvirs (Governors) consulted the Sibylline Books. Acting on the advice of the Books, they built a temple to Apollo Medicus (Apollo the Physician). Since He is Greek, the Romans built this temple outside the city. Then the priests invoked Him to come to the temple when He visited Rome. Later, families would go to the temple to seek medical attention.
To the Romans, Apollo is also the God of Divination and of Poetry. To invoke Him, poets would “drink from the waters of Castalia.” In Greek mythology, Castalia was a nymph who was pursued by Apollo. To elude Him, she threw herself into a spring. Afterwards, Apollo consecrated her and the spring to the Muses.
Personally for me, Apollo is a difficult God. I regard Him to be my “Shadow Patron.” He forces me to go places I don’t want to. Since I do write prayers and do divination, Apollo is always there. His hardness makes me into a better poet and diviner.