However you discovered them, these creatures are ripe with story possibilities. Primarily from Irish fairy lore, the pooka is a trickster character, able to take on many shapes to further his mischief. Most legends say he's particularly fond of taking on the guise of horses, goats, and rabbits (this latter form was how Harvey manifested--even if he was invisible to most).
Or, you can go by the definition from Harvey, when the orderly, Mr. Wilson, looks up Pooka in the encyclopedia:
"P-O-O-K-A. Pooka. From old Celtic mythology, a fairy spirit in animal form, always very large. The pooka appears here and there, now and then, to this one and that one. A benign but mischievous creature. Very fond of rumpots, crackpots, and how are you, Mr. Wilson?"
[Inverts and shakes the dictionary]
"'How are you, Mr. Wilson?'" Who in the encyclopedia wants to know?"
I love how this movie plays with reality and fantasy, and beliefs and happiness. The main character, Elwood, initially seems like a drunken loafer, albeit a well-dressed and garrulous one. His family certainly considers him a trial and embarrassment, but there is more to Elwood than first meets the eye.
To me, the pooka Harvey becomes a metaphor for how Elwood's nature changes, depending on whose eyes we see him through. And how the choices Elwood has made have shaped him. That can be hard to capture on the page, but when it's done well I'm always in awe of the author's skill.
It's great when even minor characters have this entire backstory--that may or may not be revealed in the course of the story--and I feel like they are completely fleshed out.
What are some books you've read recently that accomplished that?