The Wagner Tuba
It may carry the word tuba in its name but this instrument is in fact a horn; it uses the same mouthpiece as a horn and is played by traditional horn players.
The Wagner Tuba is generally used as an alternate doubling instrument to the horn and played as part of a quartet, comprising a pair of tenor tubas (played by 5th and 6th horns) and a pair of bass tubas (played by 7th and 8th horns). It's because of this doubling and quartet performance combined with the fact that there are few works written to include the instrument, that most horn players do not have a Wagner Tuba of their own.
Usually, orchestras, music colleges, opera houses, etc purchase four Wagner Tubas as a set for use as and when required. To read more about this fascinating instrument follow the links: