I guess...idk if you want to count Zekrom's tail as an ever vague indicator if its type. But idk, Lunaala always struck me as more of a Fairy/Flying mystical Pokemon than Dark. :x
The tail was one of the hints the Electric squad used, but Zekrom as an Electric type gained more prominence after the Pokémon variety show host Shoko tweeted about Zekrom being her favorite Pokémon or something along those lines. Electric is known to be her favorite type.
You know what, looking at Lunaala again, I'm starting to think it might be an Ice type. The white on it's body reminds me of ice. Also the moon cover has what looks like an aurora emanating from the moon. Though most legendries associated with the move Aurora beam happen to be psychic types...
Psychic/Ice? Ice/Flying. Psychic/Flying?
Plus I kind of associate the moon with ice in part due to it's connection to water. Hmm, might we be getting another Fire, Ice, Electric legendary trio?
Of interest is the order for them in the Pokedex if so.
Ice, Electric, Fire- Bird trio
Fire, Electric, Ice- Tao trio
Fire, Ice, Electric- Celestial body trio (possible trio, and if they follow the dex order like the Mortality trio did).
Note that Ice will be all over the place, while Electric would be the 2nd twice, and Fire the first twice.