Sir Richard Herbert was the natural son of Sir William Herbert and Maud Turberville. He married Margaret Cradock, sole heir of Sir Mathew Cradock, bringing Candleston Castle to the Herberts. Sir Richard had three children by his wife Margred (Margred, William, and George) and four by his mistress Isabel (Thomas, Alice, William, and Morgan).
Bartrum (digitization of "Welsh Genealogies AD 300- 1500" by Peter C. Bartrum)
Godwin 8(A2), Richard of Ewias (son of William, Earl of Pembroke), includes Richard's children by his wife Margred and by his mistress Isabel
"The Early Life of William Herbert, Earl of Pembroke (d. 1570)" by S.J. Gunn and W.R.B. Robinson, Welsh History Review, Vol. 18, no. 3 (June 1997), pp 509-519.
excerpt (510-511): "His father, Richard Herbert, esquire, of Ewias and Abergavenny, an illegitimate son of William Herbert, Earl of Pembroke (d. 1469), had died in 1510. Sources differ as to the exact number of children Richard fathered.... We can, however, be confident that the future Sir George Herbert of Swansea was the eldest son and heir, perhaps William's senior by as much as a dozen years." (footnote shows Sir George d 1570)