Modifying the unique placeholders/markers for a specific dragon will surely break it for other dragons. Its special design obviously doesn't fit at all the mounting/riding placeholders/markers, so it was intended IMHO. It is not supposed to be 'playable' as it is some kind of unique 'boss' linked to a scripted combat.
This specific dragon was made unique by Bethesda and appears only once in Dawnguard DLC's forgotten vale.
Also, the 2nd case you mention is not vanilla. Many have bigger necks than the serpentine dragon for example. All dragons share this unique anim set, but not all dragons are designed in the same way. As I said, anims have markers/placeholders for all your body parts and there's only 1 set for mounting/riding dragons in general. The very first dragon you can mount is a Serpentine dragon in Apocrypha, short before fighting Miraak, and everything works almost fine on it.
Mounting/riding anims on dragons came with Dragonborn.
