The story of Rishyasringa Muni is found in the Ramayana. Dashratha had three wives, but no children. So, he called a sage to perform the ritual that would get him children. The rishi selected was Rishyasringa Muni.
In Odia miniature paintings, Rishyasringa is depicted as having two horns. His father Vibhandaka was a rishi. Vibhandaka was once performing tapasya, and could not control his passions at the sight of an apsara. He spilled his semen on the ground. The semen lay on the grass and a female deer ate it. The doe then gave birth to Rishyasringa Muni, which is why he was born with two horns.
