Ovetensis episcopus Pelagius (a. 1101 – 1129) corpori chronicorum suo, de quo in praefatione ad Isidoriana disputabimus, hunc librum inseruit inscriptum teste Ambrosio Morales (apud Florez – Risco Esp. sagr. vol. 38 p. 367): incipit genealogiae totius bibliothecae ex omnibus libris veteris novique testamenti. archetypum periit, extant apographa et in codice Matritensi bibl. nat. T 10 saec . XVII (Knust Archiv 8, 799) et in Escurialensi b I 9 chart. saec. XV teste Ewaldo (neues Archiv 6, 232), scilicet tractatus inscriptus ita: incipiunt genealogiae totius bibliothecae ex omnibus libris veteris novique testamenti descriptum; adnotatur: hic liber genealogiae fuit desumptus ex libro vetustissimo ecclesiae Ovetensis in membranis litteris goticis scripto et dicitur finire in Theodosio: videtur igitur similis fuisse nostri F.The above is online at the digital MGH. It feels faintly irreverent to add hyperlinks to 19th-century Latin but I have done so, since the relevant page of Ambrosio Morales's account and tabulation in Espana Sagrada is online. Friedrich Heinrich Knust ( -1841) (biography) was a German scholar who visited archives in Spain, as did Ewald whose Reise nach Spanien im Winter von 1878 auf 1879 is also online.
As Mommsen says, the 12th-century original that was kept at Oviedo is gone for ever. Hardly any of the Escorial Library is online, so there is no point searching for images of this unique paper copy, but Antolín's catalogue is helpful.
This 12th-century copy of the Liber Genealogus seems to have been been inserted into Pelagius's Liber chronicorum. Rouse and McNelis think the final comment about "in membranis litteris goticis scripto" is by the 15th-century copyist in León, referring to Visigothic script. Or is it a note by Pelagius talking about a book that was old in his day?
It is not clear what manuscript is described in Knust's abstract ("einige Genealogien aus dem alten und neuen Testamente" in a 17th- or 18th-century codex at the Biblioteca Nacional in Madrid with the former call number T10). I did not find the Liber Genealogus listed in the other manuscripts there containing Pelagius's chronological work. There seem to be at least five manuscripts containing elements of Pelagius's Liber chronicorum at the library, including one that is a roundup with his Libro de los Testamentos de la catedral de Oviedo in Ms. 6957 (catalogue entry, warning, large PDF!). But in each case, the content at practically every folio is described, making it unlikely that the Liber Genealogus is in any gap and somehow overlooked.
The interest of the matter to me is of course: what else was lodged in the now-lost cathedral library at Oviedo in the days when Pelagius was in charge? We know the library contained the now-lost Gospel Book of Justus because Ambrosio de Morales saw it there. But did the library also contain a copy of the Great Stemma?