Boricua" goes back centuries. The island was called Boriken by the
native Taínos. "Boricua" was derived from Boriken - most likely coined by
early colonial Spaniards. All those names Boricua, Borinquen, Borincano, Borinqueño - are very old Puerto Rican terminology found in old Puerto Rican history
books, and poems that are over 200 years old, etc. If you have not studied in Puerto Rico and
read these words in our own Spanish literature (in school textbooks) and native poems then you might think of them as "foreign" but they are not. You are
right they are not "proper" Castellano, but only Spaniards speak "proper" Castellano. These terms are actually "criollo" - developed in the island. They are true Puerto Rican terminology.)