Frost Wyrm

Good guess, but no, the dragon in question in the cinematic was NOT Sapphiron. For one thing Sapphiron was never buried. Arthas killed him, stepped up to the body, and immediately put him to work. The dragon in the movie was under huge amounts of ice. It was there a while before Arthas did his thing. My theory is that you are seeing Sindragosa, a dragon known as the "Queen of Frost Wyrms." She is not currently in game but has been data mined. There is a large crater in Icecrown called Sindrigosa's Fall which looks suspiciously like the area depicted in the cinematic...

I'd rather have this be acquired through a gold/rep farm than it be a 1% drop from a raid boss. Why are you complaining?

The line of dragons (or whatever it's called) is like this:
Whelp => Drake => Dragon => Wyrm

I think this is may not be accurate. Consider Frosty being a little "wyrm" in the whelp size. So i think that the line could be actually like this:


whelp => dies => risen => Frosty
||
\=> drake => dies => risen => this mount
||
\=> dragon => dies => risen => lore-like wyrm

And there are "wyrms" in game that are definitely not big as cities at all. Remember these?

So i think there is a stage in the "line of dragons" where a risen drake can fit the look of this mount.