Original monsters means monsters that are new and written by you, so not classic enemies like a wolf, bear, skeleton, or dragon. These are something you wrote that can't be found in other lists of monsters/bestiaries. The unique monster is a single creature, there will only ever be one.
So you should have three monsters you wrote that are more of a species and you might encounter multiple different individuals (like specimens of the same species) or a group of them and one monster that has no repeats, if you encounter it multiple times it's the same individual. For example, the party might run into dozens of blood moths (one of the 3 original monsters), but only one Moth Witch (the unique) and once she's defeated they won't meet her again.
(Also, you can include classic/non-original monsters in your hexes, they just don't need a bestiary entry. So if you want roaming wolves on your map or as an encounter put them in. Whoever grabs your map can use whatever set of 'wolf' stats they want.)