I'm having this bug on 1.10

The polar bear in dynmap-mobs right now is not the Minecraft polar bear - it's actually the Mo' Creatures polar bear. You'll want to set polarbear to false in config.yml until the new mobs are supported by dynmap-mobs.

If mikeprimm accepts my pull request #15 and releases a new version you can make sure vanillapolarbear is set to true and everything should work fine.