Frogs, insects/spiders and bats have their own special rules:

Frogs can cross water, marshes, bogs or other wet ground without penalty.

Insects or Spiders are so small that they can crawl into armour and bite or sting their target. No armour saving throw is allowed if you are fighting insects or spiders.

Bats can fly, but because they are so small they may only fly up to 8" during their turn. Although they do not fly particularly fast, they can move over terrain which would slow down creatures moving over land, and they can move directly over a body of troops.