Human horn is believed by most aliens to be an aphrodisiac. It is actually a human's nose. Fry, in the episode Spanish Fry, is abducted by aliens who take his human horn and eventually end up selling it to Lrrr of Omicron Persei 8. Aliens believe it to be the human's reproductive organ - thus has aphrodisiac effects.
The "lower horn" is implied later in the episode, when Bender explains to the Omicronians that human's noses are not their sexual organs. Lrrr immediately demands that Fry give them his lower horn.