in case someone else finds this thread - each adaptive mount (e.g. Invincible, Horseman's mount, etc.) are hardcoded in server/scripts/Spells/spell_generic.cpp
to make one new adaptive mount you'll need to make 5 new spells in Spell.dbc (each speed + catchall dummy spell), reference their IDs in the aforementioned .cpp, and use spell_gen_mount() where approriate e.g. where Invincible spell handler is created.
it's a shame this relationship between various-speed mount spells can't be specified via mysql database.