After seeing how Fiddlesticks has become a horror movie monster pretty much, I'm quite curious as to what sorts of terrors it could bring to Equestria should it show up there somehow.

I wanted to say he could bring back the wendigos, but that's something born from hatred, not fear. If Fiddlesticks ends up in Equestria there might be only a couple of beings who could stop it, and by the way it feeds, entire villages would've been massacred before the ponies would have any idea. Ponies are more social than humans, so it stands to believe that if it imitated the voice of a loved one, a pony would be much more willing to go and greet it.