There seems to be a widespread genealogical myth (see, e.g. AFN:(18GK-BM1)) that there was an Agnes de Bohun, duaghter of Humphrey de Bohun, 4th Earl of Hereford (1276-1321) and his wife, Elizabeth of Rhuddlan (1282-1316), who was born in 1309 and married Robert de Ferrers, 2nd Baron Ferrers of Chartley (1309-1350), and was the mother of both his sons.
However, the standard authorities (see, e.g. Wikipedia) on these people mention no such person.
Another version given is "Agnes de Mohun (c1314-1343)".