the last truly wild horse species remaining in the world, originally found in Central Asia and first identified by the explorer [[普尔热瓦尔斯基|普爾熱瓦爾斯基|pu3 er3 re4 wa3 er3 si1 ji1]]

a wild horse that has escaped its bridle; used as a metaphor for a person or thing that has run wild or is completely out of control

To allow one's speech or thoughts to wander freely without any restraint, similar to the idiom [[天马行空|天馬行空|tian1 ma3 xing2 kong1]].