The ship was stolen by Han Solo, Chewbacca, and Kyp Durron during their escape from Kessel and Moruth Doole. Han created the name off the top of his head when he accidentally crossed paths with the Star Destroyer Gorgon in the Maw.
The Endor was created by Kevin J. Anderson and made its first and only appearance in the novel Jedi Search. It was later given an entry in The Complete Star Wars Encyclopedia, but was identified in it as being a Lambda-class shuttle, rather than a 500-X shuttle, like Jedi Search says it is.