Although it would apparently make little difference to the plot, it seems to be the case that the weasels were created some time in the past and only recently repurposed as revenant detectors. There actually is a known candidate for their creator, then: The former Spark who worked (and to a certain degree still does work) in small, furry animals, namely, Dr. Dimitri Vapnoople. Which could lead to one or more other mad theories. (On the other hand, there may be a canonical reference to the weasels as "recent" whereas Dim's loss of function is presumably not recent.)