Esperanto is the rare language named after a pen name.
Ludwik Lejzer Zamenhof was born in 1859 in Białystok, then part of the Russian Empire and now in Poland. He trained as an eye doctor. His family was Jewish and spoke Russian, and he grew up in a borderland divided by language and religion. He came to believe that a neutral international language could lower the tension between peoples.
In 1887 he published a short book setting out the grammar and vocabulary of such a language. He did not put his own name on it. The author was given as "Doktoro Esperanto," a pen name that means "one who hopes" in the language itself. Wikipedia says he called the language itself simply the international language, lingvo internacia.
The pen name did the work the title could not. Early speakers grew fond of it and began calling the language Esperanto, after its supposed author. Wikipedia gives no exact date for the change, and Etymonline's earliest English record of the word is from 1892, five years after the book.
Etymonline relates the word to Spanish esperanza, "hope," and Latin sperare, "hope." It is a fitting choice for a project that depended entirely on people deciding to take part.
The vocabulary Zamenhof chose is mostly familiar to Europeans. About 80 percent of it comes from Romance languages, with most of the rest from Germanic, Greek and Slavic ones. The name tells the same story as the grammar: it was designed to feel half-known to as many readers as possible.
It is an odd case in naming. Most languages are named for a people or a place. Esperanto is named for an attitude, and the attitude arrived first as an author's name on a title page.