The guayo is a metal scraper used in Cuban folk and popular music, most commonly associated with changüí and early son from the Guantánamo region in eastern Cuba.
According to EcuRed, guayo is the ordinary Cuban word for a kitchen grater, and the instrument closely resembles one. According to Wikipedia (also called ralladera), it is a metal cylinder with a grater surface, scraped with a metal rod or a bone. It gives a dry, metallic rasping sound.
It is the metal counterpart of the wooden güiro and produces a harsher, more percussive timbre.
The guayo belongs to the traditional music of eastern Cuba, formerly Oriente Province. It is central to changüí, a forerunner of son.
According to Wikipedia, the guayo is one of the five instruments of changüí, together with the maracas, the botija or marímbula, the bongo and the tres. Well-known guayo players are Santiago Reyes "Chago Guayo", Carlos Borromeo Planche "Cambrón" and José Andrés Rodríguez Ramírez, all three also singers in Grupo Changüí de Guantánamo.
According to Wikipedia, the guayo was largely replaced by the güiro during the 20th century and is now rare. It survives almost exclusively in changüí in the east of the country. In the Dominican Republic it went the other way: there the metal güira replaced the güiro.