As you progress through the first area you'll come across a lot of 'critters' - basically bats, rats and spiders in every dungeon. Some dungeons have 'Critters' as the main monster, so these are good ones to focus on every now and then to try get as wide a variety as possible.
The critters come in various sub-species and you'll need to face them all at least once to go into your journal. It ends up slightly confusing as you have 'Giant Spiders', 'Gigantic Spiders', 'Gigantic Venomous Spiders' and similar things.
This popped for me around the time my warriors were all on sixth level. I suspect that as your group as a whole hit sixth level it unlocks a whole new range of tiles and monsters as I started facing new Orcs too (Black Orcs). It may be the case that you need to make sure all your warriors are sixth level or above to ensure you can face the full variety of critters.