Well, it's home to Munna for one-- occasionally their evolved form Musharna as well. That is what the Dreamyard is known for, but they're typically shy Pokemon-- rare when there are too many humans about. During the time I spent there, I also saw other Psychic-types, and a few Bug-types...

[He pauses. Should he give out this information to such a headstrong trainer?] If you are kind and take care not to startle them away, perhaps you will have a chance of seeing the rarer Pokemon. They have fascinating abilities, really.