here are TWO stories ( neither of them mentioned in the Ramayana of Valmiki)
1 ) Aruni' female form.
Here is the mythological story.
Egg
Brahma's son Kashyapa had a wife called Vinata, who was the mother of birds. Once she laid two eggs. But the eggs took a long time to hatch. Impatient, she broke open one of the eggs. The child thus born became Aruni, the dawn-god, he of unknowable gender as his lower half remains unfinished.
Birth
Aruni served as charioteer of Surya, the sun-god. Aruni's female form and Surya gave birth to Sugriva. While Aruni's female form and Indra gave birth to Vali. Both the children were left in the care of Gautama and Ahilya.
Curse
Gautama and Ahilya already had a daughter called Anjana. Anjana told her father about Indra visiting Ahilya while he was away. So Ahilya cursed her to turn into a monkey. Gautama cursed the two boys to turn into monkeys as well as they had failed to inform him of the same. Later, after cursing Ahilya to turn into a rock, Gautama felt sorry for the motherless monkeys. He could not retract the curse and so he gave all three monkeys to Riksha, the childless monkey-king of Kishkindha.
2) On the peak of Sumeru Parvat there is Brahmaa's court - in 100 Yojan wide area. Once Brahmaa Jee was doing Tapasyaa, two drops of tears fell from His eyes. Brahmaa Jee took them in His hand wiped His hand with His body. Later when He shook His hand one drop fell on the ground. A Vaanar was born from that drop. He said to Vaanar - "You stay on the peak of this mountain. After some time it will be good to you."
The Vaanar went away and started living there on fruits and flowers. He used to offer flowers to Brahmaa Jee daily. Thus a long time passed. One day Riksh Raaj got thirsty so he looked for a pond, and as he bent down to drink water, he saw his shadow in that water. He thought "Who is this my enemy here? I will kill him." And he jumped into the water. But when he came out he found himself changed into a beautiful woman.
By chance both Soorya and Indra were passing from there. Both saw that beautiful woman, and had a desire to have her. Indra's semen got ejaculated and fell on the head of the woman, so a Vaanar was born from that. Because he was born from her hair, he was called Baali (Baal means hair in Hindi); while Sun's semen got ejaculated and fell on her neck, so another Vaanar was born from there and was called Sugreev (Su means good, and Greev means neck in Hindi).
Indra gave a qualitative gold necklace to Baali and Soorya appointed Hanumaan in the service of Sugreev. After this that woman again became Riksh Raaj. Thus Riksh Raaj was both their mother as well as mortal father. After some time Riksh Raaj went to Brahmaa Jee. Brahmaa Jee asked him to live in Kishkindhaa Nagaree and sent a messenger with him to appoint his elder son as the king of that Nagaree. Vaanar were already living there. So all started living there. Baali was appointed the king of Vaanar. After some time Riksh Raaj died. -
In Valmiki Ramayana, the mother of vali and sugriva is not mentioned. However, they are mentioned as sons of Indra and Surya respectively. A story which was found in other versions of ramayana states that Ahalya is their mother.
Indra and Surya transformed as Gautama and approached Ahalya.She was innocent and assumed the person to be her husband. As a result she had two sons Vali(Indra's son) and Sugriva(Surya's son).She also had a daughter with Gautama maharshi,Anjana(mother of hanuman). Gautama who knew the truth from Anjana cursed the two kids vali and sugriva to become monkeys. Ahalya too cursed her daughter Anjana to become Monkey for revealing the truth.Gautama turned Ahalya into a rock.She was relieved from the curse when Rama touched the stone with his foot.Gautama realised its very difficult for the kids to grow without their mother and as he cant revert back the curse, he gave his sons and daughter to the King of Kishkinda who didnt have kids. Anjana was married to Kesari .
Another version states that Vali and Sugriva are kids of Aruni,the female form of Aruna charioteer of Surya. They were given to Ahalya by Aruni.
surya dev was no less than indra,