@Background Pony #59CF  
Forgive me for replying to this two years late, but I’m pretty sure this is a baritone. Typically, on a tuba, there is a LOT more tubing inside the tubing loop. However, the difference between a euphonium and a baritone is that a euphonium has conical bore, which means that the tubing slowly becomes wider, even throughout the valve section, and a baritone has cylindrical bore, which means the tubing stays the same diameter throughout the entire valve section. Even though the difference in diameter is subtle between the two instruments, I don’t see any widening in the valve section tubing, so thus, I think is’ a baritone.
 
tl;dr: it’s a baritone for reasons.

She’s like Epic Sax Guy, but pony-themed.

@Background Pony #0F18  
The only practical difference between them is that the euphonium has a wider range and plays at a higher pitch than the tuba, but the two aren’t interchangeable.

@DashVader  
“Euphonium” is a euphemism for “tuba.” Or is it the other way around?

I’m honestly not sure whether that’s a euphonium or a baritone horn. If it is a euphonium, the tuba tag and “Have tuba will travel” comments are accurate, as a euphonium can also be considered a tenor tuba. The most prolific writer of euphonium music, Gustav Holst, called it that. However, baritone horns are completely different, and if that’s a baritone, it is NOT a tuba.

Quick! Somebody toss her nine more instrum–
 
…oh this was a bad idea

Have tuba, will travel!