Neutral deers and bear spawn once in a while that can be used as mounts by your units. This allows you to create mounted versions of unist that are not normally available, e.g. a bear wizard.

Waves are rebalanced so it's harder around 8 minutes and easier with bears.

Bears get downed less easily.

At least one of the rewards is a thing that you picked earlier, which allows making armies from mostly one creature type. This is overriden in late game when the number of low-level units would get too high and is replaced with a new more powerful unit type.

Pikemen do more damage.