Ribhus are mentioned in the Vedas and the Mahabharat. They are semi-divine beings. It is believed that they were originally human beings and were referred to as Ribhu Gana. Later they became immortal due to their good deeds.
Ribhus have sun-like appearance. They wear helmets and necklaces and ride on horses. The helped Indra in his adventures.
They made their old parents young again. They made prayer, sacrifice, and the two worlds. They went around the sky and after wandering for a long period, they reached the house of Savitr, who made them immortal.
One passage in the Rig Veda states that they went to the house of Agohya, and after a year, made field and grass, caused streams to flow in the right direction, and brought water into the lowlands.
Rig Veda has 11 hymns dedicated to Ribhus.
Source – The Vedas – Roshen Dalal