Why "Revantha" the youngest son of Sun god went to Vishnu's abode?
The story is told in this chapter and subsequent chapters of the Devi Bhagavatam. Surya's son Revanta, king of the Guhyakas, goes to Vaikuntha in order to see Vishnu. He comes riding on Ucchaishravas, a horse that came out of the churning of the ocean. Because Lakshmi was also born from the churning of the ocean, Ucchaishravas is her brother, so she keeps staring at it, to the point of ignoring Vishnu. So Vishnu puts a curse on her that she will live on Earth as a horse:
O Beautiful-eyed One! What you are looking at so intently? Are you so much enchanted with the sight of the horse that you are not speaking to me a single word, though I am repeatedly asking you so often! You lovingly dwell on all the objects; hence your name is Ramâ; your mind is also very restless, therefore you would be known as Chanchalâ Devî (the restless Devî). O Auspicious One! You are restless like ordinary women; you can never stay steadily for a certain time at any one place. While sitting before Me, you are enchanted with the sight of a horse; then you be born as a mare in that world of men, full of dreadful troubles, on the surface of the earth.
So Lakshmi descends to the Earth as a horse, and starts engaging in Tapasya in order to be reunited with Vishnu. Finally Vishnu comes there, also in the form of a horse, and in horse-form Vishnu and Lakshmi have a child named Ekavira, AKA Haihaya. Vishnu and Lakshmi go back to Vaikuntha, but Ekavira is adopted by Yayati's son Turvasu. Ekavira eventually becomes king himself, and because his other name is Haihaya his race become known as the Haihayas. Here is a description of Ekavira's descendants:
Then, in course of time, in the womb of Ekâvalî the King Haihaya got a son named Kritavîrya. The son of this Kritavîrya is known as Kârtavîrya. O King! Thus I have narrated to you the origin of the Haihaya dynasty.
The son of Kritavirya, Kartavirya Arjuna, was the famous king killed by Vishnu's incarnation Parashurama. And Kartavirya Arjuna is said to have been an incarnation of Vishnu's Sudarshana Chakra.
One note of caution: the Devi Bhagavatam is considered to have a lot of interpolations, and as far as I know this story isn't found in any of the Mahapuranas; in other Puranas the Haihayas are said to have been descended from another son of Yayati. So take this story with a grain of salt.