Update: So shenanigans are abound...turns out those Pokemon final evolutions are fan art...although the types of the final evolutions are valid. My bad guys!
Hats off to Pdwinnall for some awesome detective work! Thanks to him we now know what the final evolution look and types will be for two of the starters in Pokemon Sun and Moon! Litten gets taller and gets fangs and a ground type added, and Popplio gets a massive bow tie and giant fists meant for thrashing (fighting type). I still haven't seen Rowlet's evolution, but looking at these two looks like I'm going Popplio! See the reveal in the video below (action starts around 3:53 if timestamp not working).