Must it only be a plain and natural creatures? What of cave rats and giant spiders, for example?
The grunts, barks, chirps, and calls of the creatures of the wild are as language to you. You can understand any animal native to your land or akin to one whose essence you have studied.
Are cave spiders and giant rats animals native to the druid's land?
Has the druid studied their essence or the essence of a very similar creature?
If the answer to either of the above questions is 'yes' then the triggering conditions for the move are met and the Druid can understand them. Note that understanding is not necessarily the same as being able to speak to.
To answer the question-within-the-question about whether such creatures are animals or not. This is Dungeon World, a high fantasy land of orcs, goblins and dragons. Why would a cave spider or giant rat not be considered a "plain and natural creature" in such a world?
If you're really stuck, why not just ask the Druid whether they're natural animals and use his/her answer (GM Principle: Ask questions and use the answers).