Who created libretto?

Hugo von Hoffmansthal wrote libretti for Richard Strauss. Wagner and Tippett are two composers who wrote their own libretti. Usually a libretto is written before the music, but sometimes composers wrote some of the music first and then looked for words to go with it.

What period is libretto?

The style of 18th-century librettos was exemplified by Pietro Metastasio and by Apostolo Zeno, both of whom aimed at raising libretto standards by banishing comic characters from serious opera and creating a lofty poetic drama.