The mysterious Carlos Mendes was gifted with a previously unreleased Paul McCartney composition, "Penina," for an obscure 1969 single. Indeed, this is probably the most obscure composition written by a member of the Beatles, and recorded and released by someone else but not by the Beatles, while the Beatles were an active band. The song itself was a simple, even trite keyboard-based ballad that sounded more suited for the easy listening market than the rock one.
Little is known about Mendes and how he got the song, other than that he was a Portuguese vocalist and that McCartney gave it to him while the Beatle was on holiday in Portugal in late 1968. In 1994 in his Club Sandwich fan magazine, McCartney recalled, "I went to Portugal on holiday and returned to the hotel one night slightly the worse for a few drinks. There was a band playing and I ended up on the drums. The hotel was called Penina, I made up a song with that name, someone made inquiries about it and I gave it to them." The Beatles, understandably, did not record this trifle, although they did doodle around on it briefly during the January 1969 sessions for the ill-fated Get Back project. That bit has subsequently surfaced on bootlegs.
Carlos Mendes' "Penina" single was not a hit, and has been hard to acquire since then. Your best bet is looking for the EMI compilation LP The Songs Lennon and McCartney Gave Away, which has 20 songs, all but one from the 1960s, that John Lennon and Paul McCartney wrote, but that the Beatles did not record themselves for official release.