TOO LONG - DON'T READ
Not only are you allowed to explain paranormal creatures for your readers, you are pretty much required to explain them.
This explanations belong in the exposition. It’s a tautology, the exposition is where things are explained. This is normally somewhere around the first third of the book (first sixth is better).
There’s a sliding scale for what readers will accept – The closer a fictional ‘fact’ is to the beginning of the book, the easier the reader will feel about accepting it. Introducing details too close to the point where the main character needs it feels like cheating. (Ok, ok, ok, I know. This is part of American fiction writing style, not everybody does this.)
Identify the Key Points that your readers must understand about the creature for the story to work. Make sure these things are repeated, preferably with examples that will stick in the reader’s mind.
But disguise the Key Points amid other pieces of information so that the reader does not obsess about the point. AND do not give the reader too many unneeded Creature Facts so that the reader does not obsess about the point. (And NO, writing is NOT easier without readers.)
Even if you are using an absolutely bog-standard creature that Everyone Knows – You still have to define it for the reader. You just have to make sure your definition is more entertaining. If you know you are diverting from popular notions about a phantasmal creature you must address that notion. Have some character ask about the misconception, and have some other character (As you know, Bob) give the facts as they apply to your story – Or some such literary trick.
A good Dresden example is the Black Court Vampire Attack in Chapter 17 of Blood Rites. That scene is almost completely exposition – and one of the funniest fight scenes I’ve ever read (…the timer popped out…)