There are stories of Rishyasringa in the versions of Ramayana I have read, but the version in the Mahabharata attracts more.
The basic story is the story, except while Rishyasringa does not have a big role in Ramayana, post Rama's birth, his story continues for a bit more in the Mahabharata.
This is in Book 3 of the Mahabharata - Vana Prava.
A bit of the background - Yudhistira had lost his kingdom in the game of dice. Roaming in the forests with his brothers, they were wondering what to do. Arjuna had then said he will go for meditation and try to get divine weapons, which would help them in the war.
While Arjuna was away, Yudhistira had met the sage Lomasa on the Risava mountain. The mountain was so divine that only the pure heart-ed ones could see it. That is where Vishwamitra had performed his penances. That is also where a rishi, Vibhandaka had performed his meditation and had a child, born from a doe.
Curious, Yudhistira wanted to know more and Lomasa had then narrated the story.
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In the clan of rsihi Kashyapa, was born Vibhandaka, who at a young age, having proceeded towards the banks of a lake and devoted himself to penances. He lived there for years, meditating.
Once while cleaning himself in the lake, he saw the celestial nymph Urvashi. (Ramayana says Indra had sent the nymph to distract the sage, who in the form of a doe had snuggled up to the sage). Seeing the nymph, the sage was filled with passion, discharged his semen in the lake of the water. A doe that happened to be there, drank up the water, and became pregnant with child.
The doe was a cursed nymph. She had been cursed by Brahma saying she would roam around like a doe and would be freed once she gave birth to a great saint.
The child born of the doe was Rishyasringa. The doe had left her mark on the child's forehead, and there was a horn on his head.
Rishyasringa, devoted to penances, passed his days in the forest.Barring his father, he had not seen any other human and had lived his life in peace.
In the land of Anga, Lompada had once angered a rishi, who cursed the king that his kingdom would suffer from severe drought.
His priests told him to send for Rishyasringa. If he came, the rains would follow.
The king then convened a meeting of his ministers,asking for a plan for getting a visit from Rishyasringa. The ministers decided to send for the courtesans of the kingdom. Their job was to entice the saint and get him to come to the kingdom of Anga.
While all declined, fearing being cursed, an old woman in the group agreed to do the task. Taking many beautiful women, jewels, wealth, food and drink, the woman set forth towards the forest where the saint lived.
She built a floating hermitage, full of artificial trees and flowers. The trees were laden with sweets, and the artificial river was made from honey. She then had this hermitage steered towards the place where the saint lived and waited patiently.
One day she saw the saint along. She then sent her daughter to entice the saint.
Never having seen a woman before, the saint welcomed the woman. Acting as a saint herself, the woman enticed the saint to visit her hermitage. She also gave him sweets to eat, telling him these were fruits from her hermitage. She then left the saint alone.
When his father returned home, he saw his son sitting lonely. Hearing from his son what had happened, he tried to warn his son.
The next day when Vibhandaka left his ashrama, the woman came and this time she was successfully able to get the saint to leave with her.
The floating hermitage then went off to Anga in swift speed. As soon as it touched the shores of Anga, it started raining there.
The king then built roads leading to where Vibhandaka lived and welcomed the sage to Anga. In the meanwhile, he gave his daughter, Shanta to Rishyasringa as his wife. Vibhandaka was pleased to see his son happy. He blessed him and told him to leave and come back to his old ashrama, as soon as a son was born to him.
