I may be wrong, but I suspect one of the 'Vehicle driven at least once' is the Galbadian Army vehicle. To the right of 'Ragnarok', it is marked checked, and I've only ever driven two vehicles, the Car and that Galbadian Army vehicle. (For the Galbadian Missile Base mission.)

... tr("Chocobo") << tr("Hydre") << tr("???") ...
... tr("Chocobo") << tr("Hydre") << tr("Galbadian Army Vehicle") ...

I don't know French.