In 3e I used partial stat blocks for NPCs and custom monsters. They tended to be about 5 items long, every item made up from my own head. And then I ad libbed the rest.
I rustled some papers behind my DM screen whenever a new monster came on stage, and pretended like I was taking out some lengthy notes. If I needed quick access to a monster stat that I hadn't planned in advance, I made it up and made a note of it. This happened very, very rarely. None of my players ever noticed, as far as I can tell. Certainly no one ever called me on it or complained.
What's the point of this anecdote? The point is that players don't need to know how a monster was created. They just need to know what it is and what it does.
I rustled some papers behind my DM screen whenever a new monster came on stage, and pretended like I was taking out some lengthy notes. If I needed quick access to a monster stat that I hadn't planned in advance, I made it up and made a note of it. This happened very, very rarely. None of my players ever noticed, as far as I can tell. Certainly no one ever called me on it or complained.
What's the point of this anecdote? The point is that players don't need to know how a monster was created. They just need to know what it is and what it does.