Currently animals are visible in the “fog of war” (i.e. the area that's explored yet not revealed by the vision of your entities). Being able to see animals move around, run away, and get killed gives away information, information players shouldn't have. Other units aren't revealed either, not even catafalques. And you can't see changes to fields and other resources either.
This patch therefore removes the visibility of animals, improving consistency and no longer giving away information. A disadvantage is that it makes hunting harder, because you can't see in advance where the prey is, or where the wolves are.
Furthermore, this allows deleting the gaia/fauna_*_trainable.xml files.
See also D1997.