Check out this amazing interview and dragon concept art collection from Nicolas Weis! In the interview, he talks about working with Pierre-Olivier Vincent and what it was like building off the designs of the first film.
Unlike The Croods, Dragons 2 was a production oriented more on the "mechanical," whether it be war machines or for all new buildings and other stools or accessories invented by the local Leonardo da Vinci, Hiccup.
It was really a pleasure to imagine relatively seriously how to integrate enomrous flying reptiles into a human population and to cater to their needs and to their daily interactions with the Vikings. I began by making a list of all daily activities, not at all glamorous or heroic, with which had to engage a dragon when we rode.
He also designed war machines and dragon traps used by Drago's army in the second part of the film:
Originally Drago's army was divided into several factions who each had their "style" of combat. Some used sharp projectiles, some incapacitated, and others used nets. So I did a lot of research on the technique of military siege across time and cultures, then I started to sketch, make very simple 3D models, then further drawings to explain the concept before moving on to the final painting in Photoshop.
You can read the full interview on 3DVF.com (English translation)!