Affects players and animals (item_modifiers.xml).
<effect_group tiered="false">
<requirement name="EntityTagCompare" target="other" tags="player,animal"/>
<passive_effect name="EntityDamage" operation="perc_add" value="1"/>
</effect_group>
Zombie bears and dogs count as 'animal' it seems (entity_classes.xml):
<entity_class name="animalZombieDog" extends="animalWolf">
<property name="Tags" value="entity,animal,zombie,hostile,dog"/>
Tested it on a zombie bear and it does extra damage. Note that like all damage modifiers it is not exactly 200% damage with mod vs without mod. Game uses the base damage of the weapon and adds 100% of that, before other modifiers. So in my case a lvl 5 pistol went from 62 damage without to 116 damage with the mod.
The description of the mod should be changed, or zombie animals should lose the 'animal' tag. Probably that would cascade to a whole bunch of other issues, though, regarding how 'animals' are treated in game.