Of course Fimir aren't the only things that live in the marshes:
"There are countless tales of massive black-haired hounds stalking the moors in search of prey. At least some of the stories are based on the daemonic hounds summoned by the fimir as trackers or as allies in battle." - extract from the fimir army book, courtesy of the Warhammer armies project.
GW chaos hound models are absolutely spot on for the moor hounds!