Complex sentence: The poems are romances, not histories, but there is no implausibility about the use of sleuth hounds, and we can say with certainty that the dogs existed in Scotland as early as c.1300, and that their use as man-trailers was fully established.
Split into simpler sentences: The poems are romances, not histories, but there is no implausibility about the use of sleuth hounds. Thus we can say with certainty that the dogs existed in Scotland as early as c.1300, and that their use as man-trailers was fully established.