Except players or DF. It should make virtually any mob or entity Wet as long as it isn't immune to Wetness. Notable examples: Beefalo, Goat, Rock Lobster, Toadstool, Clockworks ...
Why Shadow Monsters have Wetness!?
1. Make a Waterballoon;
2. Throw it at a Mob other than Dragonfly;
3. Examine and see they are not Wet XYZ
4. Hit 'em with a Morning Star and be disappointed.