A Calthrop was a nightmarish creature, with fur and claws and fangs, a creature of snarl and snap with a gaping maw that lived in the Midlands. Its earsplitting cry was deep, savayge, and vicious and its mouth stretched wide with every roar. Its furious glowing red eyes were set deep in its skull. It had huge sharp, wet, snapping teeth. It had a blunt snout, corded muscles, and amazing strength. It was twice the size of a man.
A Calthrop had power in the night and would wait until then to kill. During the day, it could take the form of a human in order to deceive other humans whom it preyed on. In Wizard's First Rule, Richard and Kahlan encountered one disguised as an old man with a white beard and a large round belly that jiggled under his dark brown robes when he laughed. He had white hair parted neatly down the middle and long curly eyebrows and drooping lids shading his brown eyes. He had a jolly round face wrinkled with a wide smile. He called himself "Old John" and tried to trick them into believing that he was sent by Zedd, ostensibly to eat them. Although, he was run off by the appearance of Shota. A disguised Calthrop apparently had no substance to its apparent form, passing through cobwebs and by branches without disturbing them and leaving no tracks.
Side note: The Calthrop was not sent to capture them by Shota. It was simply a creature they happened upon that was waiting for nightfall so that it could devour them in its more powerful state.