RHIEINFELLT (standardized modern form)
Urien Rheged is credited with a large number of other sons besides Owein, mentioned above. One of these lines gave rise to the latest woman in our list. The contexts in which she is mentioned give a relatively high confidence-level for her historic existence. She was probably the daughter of the last Brythonic king of Rheged (Rhwyth son of Rhun son of Urien Rheged), and she is said to have married Oswy of Bernicia, possibly signaling a relatively peaceable absorption of Rheged into the English kingdom of Bernicia. Oswy was born ca. 613 (Bartrum WCD) and P.C. Bartrum suggests an approximately similar birth date for Rhieinfellt ca. 615 (ibid). She may also be mentioned in a list of queens in the Durham Liber Vitae (see below).
Nennius, wrote in Latin, roughly around 800, but he was almost certainly fluent in a Brythonic language and not only records Brythonic names in spellings that reflect familiarity with literate spelling traditions, but he sometimes records English names with Brythonic-style spellings. (For example, he writes Oswy as Osguid, and Oswald as Osguald, treating the word-internal w as if it followed Brythonic historical phonology.) He records the woman's name as Rieinmelth filia Royth filii Rum. Her grandfather's name was most likely actually Rhun (if the final nasal had, at some point in transmission, been written with a nasal suspension mark, the substitution would be understandable), and he most likely was the Rhun mentioned elsewhere by Nennius who was the son of the Urien Rheged who is honored by several poems in the Book of Taliesin (and who also figures in the context for Nyfain, Efrddyl, and Modron elsewhere in this list, q.v.) (Bartrum WCD).
The Durham Liber Vitae lists a queen named Rægnmæld (following a more English spelling system) who is likely the same woman (the identification is suggested by H.M. and N.K. Chadwick, as cited in Bartrum WCD). Jackson (LHEB p.59) dates the Liber Vitae to ca. 840, and says of some Welsh names recorded in it, "They were clearly written down for the most part by an English scribe as he heard them pronounced, and not by the owners of the names themselves as they would have spelt them." This description seems apt for Rægnmæld as well, although we must assume that both the Liber Vitae scribe and Nennius were working from existing written records, to some extent.
The name is almost certainly dithematic, with the first element equivalent to modern rhiain "maiden, queen" -- a prototheme that appears in other early feminine names, such as Rhieinwylydd (which is explicitly glossed, in a medieval text, with the Latin regina "queen"). This element originates from *riganti- (Jackson LHEB p.453). During the late 5th century lenitions, the g became a fricative (but would not be entirely lost in this context until around the 9th century). Nennius's spelling reflects its eventual loss, but this would have happened after the woman's lifetime, and in the 7th century, the g would still have been written, as we see in stone inscriptions from this period.
Similarly, we see that by the time of both written forms, the original nt had reduced completely to n. The pronunciation of this combination shifted to [nh] (or, in some contexts, a voiceless [n]) somewhere around the 8th century (Jackson LHEB p.506), but written forms in Welsh may retain nt even later. A 7th century form, however, would be expected to be written nt and quite probably to retain the full pronunciation as well.
The 7th century is roughly when we start seeing syncope (the loss of unstressed syllables) reflected in written forms (the syllables had been lost in pronunciation earlier). So we can propose a 7th century written form Rigant- and pronunciation ['ri-Gant] for the prototheme.
The deuterotheme is not as obviously identifiable, but there is a strong resemblance to the common noun mellt "lightning".
When the word-internal lenitions took place in the later part of the 5th century, m first changed to a nasalized bilabial fricative -- and was still regularly perceived as a nasal by English speakers as late as the 10th or 11th century (although it was also rendered as v starting around the 7th century), so the use of "m" in Rægnmæld doesn't undermine understanding the sound as having changed from [m]. (Similarly, Brythonic spelling traditions retained the radical when it began a distinct morpheme until a similar date.)
Both records indicate an alveolar stop at the end of the deuterotheme. Nennius's use of h may even be an attempt to include a fricative nature to the L in some way, but if so it would be among the earliest attempts to do so. This sound is more commonly believed to have developed relatively late, perhaps as late as the 10-11th century, so we need not consider it here. So we can propose a 7th century written form -melt and pronunciation roughly [B~Elt].
Thus we can suggest a Latinized written form Rigantmelta although at this point we sometimes see non-Latin names written without inflectional endings in Latin documents, so Rigantmelt is also possible. The pronunciation would be something like ['ri-Gant-'B~Elt]. Describing this in terms of English sounds is relatively difficult. Starting with something like "REE-ghahnt-VELT", the "gh" is a voiced version of the "hard ch" of Scottish "loch", the V in this case should be made with both lips and nasalized.
Koch (1997) briefly discusses this name and suggests the alternate possibility that the final -lth, rather than being a mere orthographic variant, may represent a particularly Cumbric sound-change to [lT].