Yeah, it's a double-bell euphonium.
King and Conn made them, and probably a few other companies that are defunct now. They're not made any longer and are conversation pieces pretty much, but in the right hands, these horns sing.
You notice it's got 5 valves, right? The 5th valve redirects the air column through the smaller bell, yielding a trombone-like sound. These horns were used primarily in the early part of the 20th century.
If you wanna know what one sounds like, check out this guy's recordings. He plays mostly the Holton Superbone, but also several pieces on double-bell euphonium. Alexander would play solos in which he'd have a conversation between the euph and trombone -- amazing stuff. Unfortunately, he passed far too early. A great talent.
This particular album is one of my favorites.
Ashley Alexander