Sometimes, you have time to come up with the huge nasties that dot your world. To figure out their biology, their natural enemies, what would get them in conflict with the main characters, to come up with awesome powers that make them a challenge and a description that makes everyone who so much as reads about it go “ooooo.” And sometimes, you really need a giant monster, now. So what do you do if you have to make a monster on the fly?
I usually start with one or more base creatures. Generally I’ll pick something at least somewhat appropriate to the local environment; if that fails, I’ll settle for capable of surviving in the local environment. Usually, I’ll make it a giant whatever, though sometimes it actually works better at the original size—kangaroo rats with piranha tactics, for instance, are a lot more fun when they’re rat-size rather than dog-size. Sometimes the base isn’t even a creature; I pulled one thing out of thin air based on a combination of floating candles and the flower offerings I remembered from gamelan.
Next, I try to give the creature a theme; size might make it a threat, and using multiple base creatures might make it obviously different, but that’s not going to make it fascinating. Often, the base creature inspires the theme: the floating-candle-lotus creature, for instance, was its own theme to start with. At other times, it’s themed around one of its primary powers, or around some sort of element; sometimes I just go with a really odd construction, like the time I hit my game group with a giant ghost!shipwreck!leafy sea dragon.
What about those powers? At this point, I throw in what this thing is capable of. Supernatural gifts, basic strengths/weaknesses, standard tactics, what have you, often coming out of the base creature, the theme, or both. This is done partly with an eye to challenging the group, partly with an eye to what would look really, really cool. (Let’s face it, a lot of the creatures I dredge up are mostly for show.)
Then I compile the description. The theme and the powers usually provide most of my inspiration here; if I’ve got those, all I need to do is think about what order people will notice them in, and how they can work together to look really, really cool. Typically, this won’t take more than a paragraph, unless it starts Doing Something; longer does nasty things to the pace. For extra credit, I’ll usually improvise a way to give it a grand entrance.
At that point, if I’m running game, the big advantage of it looking cool comes in; I have about five minutes of people going “Okay, that’s nifty” in which I can dredge up the rest of its stats. And, of course, if it’s impressive enough, there’s always the chance that someone will want to avoid fighting it.
End result? Nifty-looking creature with skeletal stats, usually in about ten minutes of last-minute surprise.