I love Shelgon! Don't hate on the giant bone sphere of doom! Not even joking when I say I like Shelgon a whole lot more than I like Salamence. I actually once had a shiny Shelgon that I purposely did not let evolve, I made him carry Eviolite and named him "Watermelon", he was super cool, or at least I thought he was.
Some food for thought, relating to the metamorphosis that Bagon goes through, Bag in "Bagon" is most likely refrencing what is known as a "Bagworm" which is an insect that is often known to cover itself in varying materials that may be natural from its body or natural from its environment which it eventually builds a cocoon from that it can often carry around on itself before it finds a proper spot to metamorphose into a moth.
This makes a lot of sense that Bagon would be the type of bagworm to create a carry-able coocoon from it's own skin and possibly bones over time until it is completely covered by this shell from head to toe until it is finally able to emerge from it as a full grown dragon, "Salamence".