One "untraceable terror loose in York" faced by SIGNET at some point before the Hervoken was the Galvodon. (PROSE: Hervoken Halloween [+]James Hornby, Horrors of Arcbeatle (SIGNET, Arcbeatle Press, 2023).)
Behind the scenes
The text of Hervoken Halloween does not clarify whether the Galvodon was a single being, a species, or something else.