Piccolo Flügelhorn or Sopranino Tuba
This is one of the rarest of all brass instruments, and I only include it for completeness sake. It is pitched either a fourth of fifth above the standard B-flat Flügelhorn. At one time it was used in German and Austrian bands. Today, I can only find the instrument being manufactured by Italian manufacturers under the name Flicorno Sopranino or Flicornino (Flicorno is Italian for Flügelhorn). In reality, it is very similar to an E-flat Cornet. The main difference is that the Flügelhorn usually possesses rotary valves.
The use of this small instrument might extend the range of the tuba family upwards a few notes, but its rarity makes it very difficult to score for.
A piece on Sopranino Saxhorn, which is essentially a Piccolo Flügelhorn.