The ancient Book of Jasher was a source text for both Joshua and David’s stories (Josh. 10:13, 2 Sam. 1:18). The extant version we have of the Book of Jasher, though dubitable, tells of two different stories that contain hybrid creatures that may be similar to the lion-men of Moab or the satyrs of Banias.
I don't follow Brian's logic on this point. He begins by referencing the ancient book of Jasher, then switches to "the extant version" which, by his own admission, is "dubitable."
So why does he proceed to quote it? To my knowledge, the ancient book of Jashar (or Jasher) doesn't exist at all. We don't have any extant copies or versions. What we have, instead, are fabrications.
According to one standard reference work:
An ancient writing, no longer extant, mentioned twice in the OT (Josh 10:13; 2 Sam 1:18)…The uncertain and mysterious character of the missing Book of Jashar has led to attempts to reproduce, imitate, or falsify it. An example is a late writing also entitled Book of Jashar, one of the last compositions of the haggadic literature of Judaism…Much of the material is invention, interpolated between Biblical texts, in the author’s desire to reconstruct the original book of Jashar. Many legends are added to the Biblical narrative. The account of Abraham is given in elaborate detail, including stories of his two journeys to see his son Ishmael, and of an apparition of a star. It contains a detailed explanation of the murder of Abel by Cain.
It is believed by some scholars that this attempt to reconstruct the OT Book of Jashar originated in southern Italy. The author was familiar with Italian place names, and the Arabic names in the book are due to the strong influence of Islamic culture in that region. The Zondervan Encyclopedia of the Bible (Zondervan, rev. ed., 2009), 3:469.
Brian references a self-published edition by some guy named Ken Johnson, whose credentials are elusive. This source has zero credibility.