1: [Young]
Clearly the Bowgart kid is outsized by its instrument. Until it can grow full horns, it'll have to make do with hand-me-downs. It's a good thing that a Bowgart grows two horns; a younger's first attempt at making an instrument is not always successful.
2: [Adult]
Age and experience turn Bowgarts into skilled luthiers. They always have a collection of spare instruments around. It is generally considered wise to leave one's extra instrument unstrung though, as curious novices tend to produce a sound that can only be described as ghastly.
3: [Prismatic]
This Bowgart returned from the Prism Gate forever changed! The Monster's limbs, previously specialized to suck mud up through its feet and multitask in the playing of its instrument, have been modified (and multiplied!) at the molecular level to instead produce webbing and serve as organic bows and strings - extraordinary!