One of The Legend of Zelda: Breath of the Wild's biggest mysteries centers around the origin and true nature of the enigmatic and dangerous Guardians. The game's most infamous enemy by a wide margin, the Guardians were first introduced in BOTW's original teaser trailer back in 2014. The creature is sometimes stationary, other times moving on its own, but is always waiting for Link to pass too close so it can incinerate him with its laser eye attack.
The Guardian is a large creature in The Legend of Zelda: Breath of the Wild, but that doesn't make them any less dangerous. They're incredibly fast, and even if the player manages to get a good distance on them, their laser attack will quickly close that distance for them. They're often guarding Sheikah towers, Zelda: Breath of the Wild's many hidden secrets, or are just placed randomly so that the player can't get too comfortable when traveling.
One of the secrets to why the Guardians are so intimidating in Breath of the Wild is the fact that the player is told precious little about them in terms of who built them and why - at least at first. The secret origin of the Guardians and the other Sheikah artifacts are things the player has to go looking for, just like everything else in this game. But the answers are there, if the right conditions are met.
BOTW - Flashbacks To Link's Past Reveal The Guardians' Origins
Throughout Breath of the Wild, the player will find spots that give Link a bit of his lost memories back, giving greater insight to the context of what's going on in the story. This sidequest is how the player learns about Link's old comrades, his relationship with Princess Zelda, and of course, the nature of Zelda: BOTW's Guardians themselves. Not only is this how the player is told how Hyrule interacted with the Guardians, and why they're always at very select - almost strategic - locations, but who built them and why.
The Guardians hail from Hyrule's distant past, created by the Hylians of 10,000 years ago with the sole purpose of stopping Ganon in the event of his resurrection. While the Sheikah shrines were built to provide the Link of each generation with the means to train to his peak, the Guardians were built to give him and Zelda an army. After defeating their generation's Ganon, the Guardians were lost to time, being unearthed 10,000 years later during Breath of the Wild's story. When Ganon returned, however, he took control of both the Guardians and the massive Divine Beasts, turning them on Hyrule and transforming them into the monsters players live in fear of throughout the game.
With Ganon's defeat at the end of Breath of the Wild, it's unclear what becomes of the Guardians, whether they just turn off or go back t0 being on the side of good. It would be interesting seeing them either way in Breath of the Wild 2, either as eerie lifeless husks or proper protectors like they were meant to be. Whatever the future may hold for them, the lore surrounding them and their role in the game makes the Guardians one of The Legend of Zelda: Breath of the Wild's most interesting elements.