Horn players can play the Wagner tuba, despite the name
Wagner really liked the sound of the tuba. But the tuba's tone is too heavy to play the melody, so he conceived of a hybrid instrument half way between a horn and a tuba. This is called the Wagner tuba, and its resonant timbre can be heard in the musical drama The Ring of the Nibelung. The Wagner tuba was also employed in Stravinsky's and Bruckner's music.
The name includes the word "tuba," but the timbre and style of playing is similar to the horn, and in modern orchestra's the horn players are tasked with playing this instrument.