I thought the trio was all based on different areas of Yggdrasil (Yveltal = the eagle at the top, Xerneas = the stags around the trunk, Zygarde = Jormungand, the serpent around the roots).
I can see 10% ZyDog being based on Fenrir, but I just find it a bit silly that it looks mostly like a regular doberman, but green. I'm assuming Perfect Zygarde is based on Hel, being half alive (X) and half dead (Y). ... I got nothing for the blobs.