I saw this in a parade.
The tone sounded like a horn. I guess it is enlarged to make it louder in open areas.
It's a Sousaphone. Invented by one Philip Sousa, famed for his marches. He wrote many, which are still played, particularly in USA. He needed an instrument that would be capable of playing the lowest notes in marching bands, and came up with the serpentine bass you see.
Many years ago, I had one, sadly sold it to a friend. It took hours to polish, but later versions are fibreglass, so easier to maintain - and don't mind too much being played in the rain!