Most of the monsters I am using now are simply made up, either based on something from Appendix N, something from real life, or something that I thought of to meet a particular need. Or all three smushed together.
When there is an entry in the Core Rulebook that lists something close, I tend to use that as the basis, but I am not adverse to simply striking out on my own. I think that the coolest monsters break the rules in some way. Not only do they do something unexpected, but there is sometimes an awful consequence to not defeating them. Something worse than death, as often as I am able to pull it off. "Well, no, your character survives, but....."
Not every monster, obviously, but it is nice to have something in a scenario that can truly mess with the PCs should they fail to defeat it.
I have Raggi's
Random Esoteric Creature Generator, but I have yet to refer to it. I also picked up
The Metamorphica, although I have yet to refer to that, either. It is always nice to have idea generators for when you want them, and I recommend both of these. I am not using them yet, but I have looked through them, and both are well worth purchasing.
I think Joseph Goodman was right. Don't let a monster book limit you.
By all means, use any and all monster books available, for any edition and any game system -- to get ideas, for guidance on how to stat monsters, for inspiration -- but use them as the starting point only.
RC
