If those monsters are trademarked by Wizards of the Coast (and I think they are), then yes, they might come after you to pay royalties. It doesn't matter that your game is free; you'd be using someone else's intellectual property. That's unfair and nonsensical, but sadly it's how the law works in most countries nowadays. At least file off the serial numbers. Name them something else and change some cosmetic details. It's how they derived halflings from hobbits originally after all.