In my opinion the best counter vs Horrors is Astral Geysir. Of course you need access to Astral mages for it, but if you have them, then your opponent will attempt exactly one time to use Horrors against you. It is not nice to be eaten by Horrors you yourself called
Plus AI seems to like to target horrors with the Geysir, so the horrors end up fighting themselves (always a nice view to watch :P).