Thomas of Erceldoune, otherwise Thomas the Rhymer, and in the popular style True Thomas, has had a fame as a seer, which, though progressively narrowed, is, after the lapse of nearly or quite six centuries, far from being extinguished. The common people throughout the whole of Scotland, according to Mr Robert Chambers (1870), continue to regard him with veneration, and to preserve a great number of his prophetic sayings, which they habitually seek to connect with "dear years" and other notable public events. A prediction of Thomas of Erceldoune's is recorded in a manuscript which is put at a date before 1320, and he is referred to with other soothsayers in the Scalacronica, a French chronicle of English history begun in 1355.
The authoritative reference is An Introduction to the Romance and Prophecies of Thomas of Erceldoune, 1875, by lexicographer James Augustus Henry Murray, general editor of the New English Dictionary on Historical Principles (the OED).
Sir Tristrem (text and introduction). Sir Walter Scott, the first editor of the Middle English poem Sir Tristrem, believed it to be written by Thomas of Erceldoune. The poem survives only in an anthology of Middle English literature known as the Auchinleck Manuscript, rescued by Alexander Boswell of Auchinleck (the father of James Boswell, Samuel Johnson's biographer) "...in 1740 from a professor of Aberdeen University who had been tearing out leaves to make covers for notebooks." This introduction to the metrical romance by Alan Lupack at the Camelot Project discusses the question of dialect and attribution. New !
Sir Tristrem from the Auchinleck manuscript (slow to load; be patient!) New !
Thomas Rymer, the ballad with textual history and commentary, from The English and Scottish Popular Ballads edited by Francis James Child, at Tam-Lin.org.