Anyway, shapeshifting works best if there is someone who can damn/blind/web mob and heal you, even if that someone do not enters fight. So let him be riding
. Just imagine - huge fierce dragon and a beautiful female bard/cleric on his back - it's a classic picture . Seriously, I don't see any reason why this can't be done. Surely, right now there are some bugs when tranportation spells are used while mounted, but that needs to be fixed anyway. And if you don't want for someone to mount you against your will you can allow mounting only to a person you follow. If that was me I would give mountability to form starting from Roc or even Griffon, leaving Crocodile and Naga (mythic forms probably should be left unmountable too - I mean if equanimity doesn't work with them, why would they allow someone on their back?).