A Thunderclast is an "enormous stone beast" from the times of the Desolations.
The Prelude to The Stormlight Archive describes one of these beasts as "vaguely skeletal in shape, with unnaturally long limbs that sprouted from granite shoulders. The eyes were deep red spots on the arrowhead face." It had "riblike protrusions from its chest" and its "hand was as long as a man was tall." The beasts had "ripped themselves free of the stone to join the fray" of Aharietiam: the last Desolation.
Dalinar sees a live Thunderclast during his vision of a hunt on the Purelake. The creature was animated by a spren "like a full figure, easily six feet. A swimming person, but like a shadow." The spren dives into the rocky lake bottom and the Thunderclast rips itself from the stone. The creature's body is described as "skeletal, with thin bony limbs and spiky fingers that ended in rocky claws. The chest was a rib cage of stone." It stood thirty feet tall.