Do none of you even third movie?
They're ancient pokemon that reside in either GIratina's dimension, or one similar. They inspired writing (if you use the games only as canon, western writing. If you include the anime, then all writing consisting of an alphabet).
The psychic typing is most likely symbolic as they grow more powerful when more of them gather (letters -> words -> paragraphs, etc) and they fact they can create illusions dreams (stories). Or it could be attributed to the fact they come from another dimension.
That's about all that can be said about them. There hasn't been any information, that I know of at least, about why they can cross dimensions or anything.
