"A mysterious bridle made with forgotten ancient technology. Equipping a horse with this will maximize their abilities, increasing the number of spurs associated with them."
The Ancient Bridle is a piece of Horse Armor which appears in Breath of the Wild and was released as a part of the second DLC expansion for the game: 'The Champions' Ballad'. The Bridle, along with the Ancient Saddle, are acquired as part of the EX Ancient Horse Rumors side quest. When equipped, it increases the spurs available when riding by two. However, some horses, such as the Giant Horse or Epona, are unable to wear the Ancient Bridle.
Over at the Highland Stable, Link will find the Super Rumor Mill V3 on the table, which hints at the location of the Ancient Bridle. The rumor mill give Link the hint that the Ancient Bridle is located south of Hyrule Ridge, beneath a cherry blossom tree at a mountain. The hint refers to Satori Mountain where there is a cherry tree just on the north side of the peak. Use Magnesis to pull the treasure chest out of the ground and get the Ancient Bridle.
Check out the Interactive Map for a more precise location.