Pengolodh was a scribe, loremaster and linguist born in Nevrast, surviving the fall of Gondolin, and sailing to EressŰa late in the SecondAge as Sauron won dominion over Middle-earth.
The spellings Pengolod and Pengolo occur frequently. The Quenya forms of his name is given as Quengoldo and Quendingoldo[HoMeXII]. The Sindarin name Thingˇdhel is a "late proposed subsitution" for Pengolodh [HoMeXI].
Like most aspects of the mythology, Pengolodh evolved a good deal throughout the years. His origins might be found in Gilfanon of Travrobel, Christopher Tolkien remarks [HoMeIV]. The first occurance of "Pengolodh" was several years after the LostTales, in the preamble to the Earliest Annals of Valinor one exiled Noldo Pengolodh is initially credited with having written them [HoMeIV].
Sortly thereafter R˙mil would be the chief writer of the Annals of Valinor and the later Annals of Aman, while Pengolodh only added to them [HoMeIV, HoMeV, HoMeX].
The AinulindalŰ, though written by R˙mil, was at one point to have been spoken to Ălfwine by Pengolodh [HoMeX]. Whenever Ălfwine is concerned, it was Pengolodh who, working with the lore of sages before him, served as the primary source of information for the man, though the details of his exact role change over time.
At one point in time it was Pengolodh himself who was the author of the Quenta Silmarillion, and from him that Ălfwine translated the work into his own tongue [HoMeV].
In addition to the changing role of Pengolodh with regards to the internal origin of the Silmarillion, the elf's history itself had gradually changed. Eventually he was said to have originated in Nevrast instead of Valinor, and to be of Sindarin and Noldorin ancestory. In any case it is thanks to his rescuing written lore (both ancient and some of his own compilations) from the destruction of Gondolin and to his "prodigious" memory, that much of the matter of the Elder days had been preserved.[HoMeXI]
Pengolodh is not only mentioned throughout The History of Middle-earth where the source of annals or the Silmarillion is concerned, but it is to him that much of the linguistic essays published in HoMe have been attributed. Some works featuring, drawn from, or recording the the lore of Pengolodh are: Dangweth Pengolo, Quendi and Eldar, and The Lhammas.
It is worth mentioning that Pengolodh lived among the Dwarves in Khazad-dűm for some time, and learned their language. He was the most prominant member of a school of linguists called the Lambengolmor "Loremasters of tongues" since FŰanor who founded it.[HoMeXI]