Compare Old Portuguese ventãa 'nostril', Modern Portuguese venta 'nostril of animals', Galician béntas 'nostrils of a horse'. DRAE, Corominas 601, Corominas, Pascual V, 806-807, REW 9212. Spanish ventana as 'an opening in the wall' since 1400, earlier only 'air hole (of ship, of marquee, tabernacle, of armour)' since 1250, 'nostril' (attested on 1325). Semantic widening of Spanish ventana 'air hole, ventilator window'→ 'window' is explained by phonetic coincidence of two Old Spanish words hiniestra 'window' (from Latin fenestra) and iniest(r)a 'greenweed' (from Latin genestra). Meriger supposed semantic influence of Visigothic *windaugo (compare Old Norse) but Meyer-Lübke in REW doubted this because of the French ventail 'air hole of helmet' which arose independently from Visigothic.
'air hole, ''opening for wind'' → 'window'. Mongolian realisation demonstrates the initial stage of this evolution.