SB 11.4.17
hamsa-svarupy avadad acyuta atma-yogam
dattah kumara rsabho bhagavan pita nah
visnuh sivaya jagatam kalayavatirnas
tenahrta madhu-bhida srutayo hayasye
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The infallible Supreme Personality of Godhead, Visnu, has descended into this world by His various partial incarnations such as Lord Hamsa [the swan], Dattatreya, the four Kumaras and our own father, the mighty Rsabhadeva. By such incarnations, the Lord teaches the science of self-realization for the benefit of the whole universe. In His appearance as Hayagriva He killed the demon Madhu and thus brought the Vedas back from the hellish planet Patalaloka.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It is stated in the Skanda Purana that the Lord of the universe, Hari Himself, once appeared in the form of a young brahmacari named Kumara and spoke transcendental knowledge to Sanat-kumara.