Shardik is one of the twelve guardians of Beams that hold up the Dark Tower. It is a cyborg bear that stands over forty feet tall and has a contraption affixed to its head resembling a satellite dish (known as a thinking cap by Roland).
Shardik was built by North Central Positronics, the mysterious company which was a part of The Imperium thousands of years prior to the events of The Dark Tower Series. Shardik was known as Mir by tribal groups who once lived in the woods of his domain, which in their language means, the world beneath the world. Shardik's functional longevity is most likely due its internal nuclear power source.
It is possible that Shardik might have had a non-cybernetic form apart from the one encountered by Roland and his Ka-tet: Maturin, the turtle beam guardian, is revealed to be a force of the white, and aids Roland and his allies on several occasions, and is revealed to be a powerful "macroversal" spirit in It.
Left on Shardik by the Old Ones is the following:
DESIGN 4 GUARDIAN
SERIAL # AA 24123 CX 755431297 L 14
NR SUBNUCLEAR CELLS MUST NOT
BE REPLACED NR
The Waste Lands
After several millennia of service watching over the beam it had begun to deteriorate and die. Biologically, the bear suffered an extensive infestation of worms. Electronically, the bearings in its "thinking cap" were worn and, according to Eddie, about to freeze up, presumably resulting in the creature's death.
When Roland and his ka-tet arrive and set up temporary residence in Shardik's woods, the creature becomes visibly upset. Thinking them to be the "troublesome fire starters" who lived in his woods years past (delusions presumably brought on by extreme fever and madness), he decides to kill them.
Shardik runs to Roland's camp in the clearing to find Eddie. He chases Eddie into a tree and attempts to shake Eddie loose. In the first real test of her newly-learned gunslinger skills Susannah Dean has to shoot the metal "thinking cap" on the bear's head. She succeeds and as a result, kills the bear.
Post-mortem inspection reveals to the ka-tet that the bear is indeed one of the Guardians of the Beams. This, as revealed to the reader, is what Roland has been looking for so long. Following the creature's back trail they find the beginning of the beam and start on their final path to the Dark Tower.