The troll has to be a very dimensional creature. He is part of the lore of this land. And this particular troll has been set for reasons unknown to us, to gaurd this particular bridge. He is the gatekeeper to from one part of this world to another.
But set there how? and by whom? And just how old is he? Did he build the bridge himself and if so it is enchanted?
As I did him I began to think that all these things might be true, but also that he had begun to make the best of his odd situation. Maybe he was placed here as a curse long ago when he lost a contest to a brave and daring, wise king. And he must now live forever under this bridge. Or maybe he took up residence here when he found that he could exact a horrible toll from all who needed to cross. Now he wayalys any and all travellers on the king's highway.
And maybe he has found that he has a decided liking for Lamb.
The sheep have eaten all the good grass on the one side bridge and seeing the beautiful lush grass on the other cant help but try and cross over. The troll has the brightness to realize that this is good for him. He can pick them off one by one as they cross and having himself a merry feast every night.