These two sons of John fitz Gilbert de Monmouth, Lord of Monmouth have been confused with each other by many historians. The following seems to be the clearest way to disambiguate them:
Apparently John FitzGilbert of Monmouth had two sons named John, one from each marriage. The elder son John of Monmouth John (the Elder Brother) de Monmouth, Lord of Monmouth (born of his first wife Cecily de Waleran, probably in the 1190s) was custos of the castle of Penrhos and died in 1257 without issue, leaving Monmouth Castle to Prince Edward.
That elder son John is to be distinguished from his younger half-brother John John II de Monmouth Lord of Monmouth, born to his father's second wife Agnes de Muscegros around 1225. According to an article by Crowley et al., citing Rotulae Parliamentariae i, 185, the elder John left property to his younger half-brother, but the younger John de Monmouth was hanged for murder. Another report was that he became heavily in debt and surrendered his estates, including the lordship of Monmouth, to the crown in 1256. [from http://en.wikipedia.org/wiki/John_of_Monmouth] He died in 1274, or according to Crowley et al, in 1281 However, this conflicts with the account that his brother had left Monmouth Castle to Prince Edward, so it seems that historians may have often confused the two Johns.
Crowley et al. only discuss the dispensation of the parish of Steeple Langford, but through their discussion it appears that the older brother John apparently left his estates to the younger brother (and perhaps to a number of siblings), and that it was not that he left his estates to the crown but that they were seized by the crown and later returned to the family after petitions from various heirs:
"Osulf held Steeple Langford in 1066; Waleran the huntsman held it as 10 hides in 1086. (fn. 57) The manor of STEEPLE LANGFORD descended to Waleran's heirs, possibly in the direct male line and presumably to William son of Waleran, Waleran (fl. 1131) son of William, Walter Waleran (fl. 1166), and Walter Waleran (d. 1200–1). (fn. 58) The second Walter's heirs were his daughters Cecily, Aubrey, and Isabel; Cecily married John of Monmouth, Aubrey Sir John de Ingham (d. c. 1203) and William de Botreaux (d. c. 1209), and Isabel William de Neville. The husbands held the manor jointly in the early 13 th century, (fn. 59) and John of Monmouth, Aubrey de Botreaux, and Isabel's daughter Joan de Neville (d. c. 1263), who married Jordan de St. Martin (d. c. 1223), held it jointly and in chief in 1242–3. (fn. 60) John of Monmouth (d. c. 1248) and Cecily were succeeded by their son John (d. 1257) who settled the manor or his right in it on his brother, evidently half-brother, John of Monmouth. (fn. 61) The brother was hanged for murder in 1281. (fn. 62) Aubrey (d. c. 1270) had a son Walter de Ingham (d. c. 1253) and a grandson and heir Oliver de Ingham (d. 1282), (fn. 63) and Joan had a son William de St. Martin (d. c. 1291). (fn. 64) Oliver and William claimed the manor but Edward I took it as an escheat, asserting that John of Monmouth (d. 1281) held it in chief, (fn. 65) and in 1299 settled it as dower on Queen Margaret. (fn. 66) In 1304 Oliver's son Sir John de Ingham and William's son Reynold de St. Martin petitioned parliament for the manor, acknowledging the king's right to have held, it for a year and a day but claiming it as their escheat on the grounds that John of Monmouth (d. 1281) held it not in chief but of the heirs of his brother John as coparceners. (fn. 67) In 1306 they recovered seisin, (fn. 68) and by 1310 had partitioned the manor, (fn. 69) which thereafter descended in moieties until 1588."
Footnotes/references for this last quote:
57 V.C.H. Wilts. ii, p. 151.
58 V.C.H. Hants, iv. 351, 521; I. J. Sanders, Eng. Baronies, 96, where Walter (d. 1200–1) is said to be the son of Waleran (fl. 1131); Hoare, Mod. Wilts. Cawden, 73; Red Bk. Exch. (Rolls Ser.), i. 241.
59 Red Bk. Exch. (Rolls Ser.), i. 153–4; ii. 483; V.C.H. Hants, iv. 521; Sanders, Eng. Baronies, 96–7, which this acct. of Walter Waleran's successors corrects at several points.
60 Cal. Inq. p.m. i, p. 166; Ex. e Rot. Fin. i. 100; Bk. of Fees, ii. 716; P.R.O., CP 40/355, rot. 89 and d.
61 Sanders, Eng. Baronies, 96; Cal. Inq. p.m. i, p. 101; Close R. 1256–9, 63; P.R.O., CP 40/355, rot. 89 and d.
62 Rot. Parl. i. 185.
63 Cal. Inq. p.m. i, p. 232; Complete Peerage, vii. 65 n.
64 Cal. Inq. p.m. i, p. 166; Cal. Fine R. 1272–1307, 295; P.R.O., CP 40/355, rot. 89 and d.
65 Cal. Fine R. 1272–1307, 185; P.R.O., CP 40/60, rot. 3; CP 40/355, rot. 89 and d.
66 Cal. Pat. 1292–1301, 452.
67 Rot. Parl. i. 168, 184–7.
68 Cal. Pat. 1301–7, 505.
References
http://en.wikipedia.org/wiki/John_of_Monmouth
http://en.wikipedia.org/wiki/John_of_Monmouth_(died_1257)
Kissack, Keith (1974). Mediaeval Monmouth. The Monmouth Historical and Educational Trust. pp. 24–27.
John Duncumb et al., Collections Towards the History and Antiquities of the County of Hereford: pt. 1., E.G. Wright, 1812, pp.368-369
D. A. Crowley (editor), A. P. Baggs, Jane Freeman and Janet H. Stevenson (1995). "Parishes: Steeple Langford". A History of the County of Wiltshire: Volume 15: Amesbury hundred, Branch and Dole hundred. Institute of Historical Research. Retrieved 12 May 2012. http://www.british-history.ac.uk/report.aspx?compid=115439