The Bridge Troll is an original character from The Land of Stories series, though this kind of "bridge / river guardian" type character is common in many folk tales. He appears in The Wishing Spell, ch 9 only, and refuses to let Alex and Conner over his bridge unless they solve a riddle. His bridge is between the Corner Kingdom and the Charming Kingdom.
Appearance and Personality
"A large troll had jumped right in front of them on the bridge. He was short and very wide with an enormous head. He was covered in matted fur with large eyes and a snout. His arms and legs were tiny, but his nails and teeth were sharp and long." 
The troll threatens to eat them if they get the answer wrong, but does not attack them when Conner chooses to cross the river without using the bridge, and Alex solves his riddle correctly.
"If you wish to cross my bridge, you must answer my riddle correctly!" the bridge troll said. "Answer incorrectly, and I'll bite off your heads."