If ya look close in the trailer, you can see Fenniken use what looks like Psybeam, Chepsun use what I believe to be Furry Swipe, and Foroky looks like he might have used some type of fighting move. This leads me to deduce that each Pokemon's final evolution will each be multitype, Fenniken being Fire Psychic, Chepsun being Grass Dark, and Foroky being Water Fighting. This also gives a type advantage wheel that works in reverse so that each starter may have a chance against one another.
Yeah, honestly I really doubt they can shift Fenniken's form and more so that it actually looks like it could work as one. Not to mention have a reverse type triangle of Fighting/Psychic/Dark working along with with the classic Water/Fire/Grass makes that starters a much more balanced bunch. And I hope it is to!
Zen mode yes but a Darmanitan will only change that form in times of crisis. In its regular standard form, a darmanitan's true attribute and category is that of a pure fire type. Victini on the other hand, has both the attributes of a fire and a psychic type in it's standard form.
So far there are currently 8 poketrivias (excluding the cover and the New Year's art), the first five of which are all about poke balls while the other 3 (which includes this one) are about random topics related to pokemon. If you want to check out the folder, then here's the link