Like 90% of things you might consider a beast count. If it has beast in the name, it probably counts, even Demonbeast Victoria and Moon beast but not forbidden beast behemoth or Lingeriena (who doesn't have beast in her name but you'd think she would be affected since Victoria is). Fish may or may not count. For some reason, none of the 3 nightmares count, nor Node.
For most standard enemies it is only 20% bonus damage. For headhunting beast (and most other beast bosses) it is 50% bonus. Most enemies have an elemental weakness though (usually 50% bonus) and you can only target one weakness at a time so if you are using resins, you won't notice a difference in damage.
