Guardians are enemies in The Legend of Zelda: Breath of the Wild.
The Guardians were originally created by the Sheikah ten thousands years before the events of Breath of the Wild along with the four Divine Beasts, from their ancient technology, to protect the Hyrule and aid the Princess of Hyrule and the Hero to defeat Calamity Ganon and sealed him. The Sheikah and their technology was firstly hailed as the power of the gods, but as time went on the people and Royal Family of Hyrule came to fear their power as a threat to the kingdom, resulting in the Sheikah becoming exiles.
One hundred years before the events of the game, King Rhoam Bosphoramus Hyrule became aware of the prophecy about the inevitable return of Calamity Ganon. This drove a search for the fabled ancient Sheikah relics. Upon their discovery, King Rhoam decided to excavate them. He then formed a group of skilled Champions commanded by Princess Zelda, and four elite warriors were selected from across the kingdom and tasked with the duty of piloting the Divine Beasts to assist Link, the appointed Knight and Hylian Champion. One hundred years prior to the events of Breath of the Wild, while the Champions were making preparations to battle Calamity Ganon, he reappeared and devastated the Kingdom of Hyrule by taking control of the Guardians and the Divine Beasts, killing four of the Champions in the process. A century after the Great Calamity, Link awakens from his slumber in the Shrine of Resurrection he first encounters remaining Decayed Guardians at the Eastern Abbey. He then finds more Guardian variants during his adventure.