Anything outdoors and more than 2ft away from the internet fills me with cold shivers and the sweats
I was going to say more than ten yards from the glow of the fire.
If not actually due to the terrain itself...
Often, very inconsiderately, these animals will opt to jump across voids rather than go around. These can be little ditches, or thousand foot cliffs.
I have only ever seen one chap getting dangerously stuck in a bog, but picking a safe path across an area of bog is tricky at the least.
Goes without saying, any terrain covered in the stuff is many times more tricky, but the kind of trails that traverse near vertical slopes, with falls on the unlucky side, become a whole different thing once ice is a factor.
There is a strong reason to fear the unknown, and the combination of tiredness and the unknown has done for many. A benign looking headland with established paths and pleasant views can, in an instant, become your ending place as the turf is all that is left in the formation of a blowhole, for example.
As far as surfaces I don't care for riding on, rather than actual terrain, you can add wet mud, slime, ectoplasm, dead leaves, rotting seaweed and loose rock on any kind of slope for anything more than a short stretch to what would probably be a very long list.
The doctor diagnosed me with hereditary diarrhoea. I can't understand, I wear a kilt.