Proto-Mongolian: *unagan
Meaning: foal up to one year old
Russian meaning: жеребенок до года
Written Mongolian: unaɣa(n) (L 875)
Khalkha: unaga
Buriat: unaga(n)
Kalmuck: unǝɣṇ (КРС)
Ordos: unaGa(n)
Dagur: unek 'kid' (Тод. Даг. 171)
Shary-Yoghur: naGan
Monguor: naGa (SM 254)
Comments: MGCD 674. Mong. > Evk. unukān etc., see Doerfer MT 104, Rozycki 218. Associating *una-ga(n) with una- 'to fall, drop' is most certainly a folk etymology. -gan is a widely spread suffix in animal names (daɣa-ga(n) 'colt', qura-ga(n) 'lamb' etc.); the usage of una-gan as 'having certain characteristics from birth' is also easily explainable: cf. phrases like unaɣan ǯiruɣa 'a horse able to travel at an amble since birth', lit. 'foal-ambler' = 'fast ambler since foalhood'.
