Never sent anyone on a troll hunt, but I did have one game that involved the party following a glowing sheep through the sewers
1st ed Warhammer, starting characters, so pretty weedy. To earn some cash they were hired by a freakshow owner to track down an escaped 3 legged goblin. It had vanished into the sewers under whatever Empire city they were in. Of course, there were rumours of some horrible monster in the sewers as well, so they hit on the idea of getting a sheep on a rope, and casting a light spell on it.
This would reduce the need for a lantern, and if there was something horrible, hopefully it would attack the sheep first, giving the players time to evaluate it and either attack it or run like hell.
I don't think they found the monster, but they did find the remains of the goblin. Apparently it found the monster, judging by the teeth marks. At that point they decided running away was the better part of valor :)