I think 10 % Zygarde's (Canine) learns Thousand Arrows, it howls and shoots arrow to Yveltal. While 100% Zygarde shakes the ground with Thousand Waves to trap Xerneas.
The only thing that connects Zygarde's forms is they are all based of the Norse Mythology.
https://norse-mythology.org/gods-and-creatures/giants/fenrir/ (canine)
https://norse-mythology.org/gods-and-creatures/giants/nidhogg/ (The Zygarde as we first know)
https://norse-mythology.org/gods-and-creatures/giants/surt/
Both are gigantic humanoid beings, Surtr is known for being charred, while 100% Zygarde is mainly dark.
Surtr is notably a guardian/ruler of Muspelheim and Zygarde's a hidden guardian of the Kalos region.
Surtr's arrived from Muspelheim, a land of extreme heat and fire. While Terminus Cave isn't stated to be particularly hot, it's worth noting that its location doesn't snow like Snowbelle and Anistar City.
The Ragnarok event might indicate that Kalos will be torn apart by both Xerneas and Yveltal, with Zygarde stepping in.