Ok, I know I write fantasy and a lot of leeway has to be given, but let’s play with making a monster real for a moment. I’ll take the Giant Spider because I’ve spent a lot of time thinking about this one. Hay, I’m not slamming your hobbies, am I? :-)
Actually, I spent a lot of time thinking of how to up-scale anthropoids when I wrote War of the Worlds 2030 since the alien invaders used genetically modified earth life forms as their troops.
Bugs make good monsters in large part because of their psychological effect on humans. They seem almost like aliens on our own world with their body plan and exoskeleton.
A lot more goes into making a giant spider viable than just upsizing it though. First you hit the oxygen transfer limit. Anthropoids breathe through their skin as such they can’t get too large and still supply the oxygen needs of their body cores. If you want to upscale a spider, an arachnid, part of the anthropoid group, you have to deal with this. More oxygen in the atmosphere is one way, which was the case in the Devonian period, adding some kind of a lung or blow through vents to the body is another.
Next you run into the limitations of an exoskeleton. An exoskeleton may be built in armour, but it is not as useful for supporting a creature structurally as the solid pole construction of an endoskeleton. So if you want a giant spider you must A – reinforce the material of the skeleton. This process is useful in fantasy because it makes a more powerful beast. Or B – add an endoskeleton. This would make cutting off limbs even harder.
You might want to look at adding additional hearts when you upscale a spider as well to keep the blood flowing.
In the Fate of the Norns Ragnarok gaming system the giant spider is highly intelligent, which means a large, complex brain. The thing to keep in mind is that for life on earth the size of the brain is less important that the ratio of brain size to body size.
A T-Rex had a fairly large brain, but in relationship to its body size it wasn’t that impressive.
This is only a start. When making monsters for fantasy and books a little understanding of biology coupled with a lot of imagination and problem solving is a very useful mix for getting it right. One thing that often bugs me on the show Face Off, I really enjoy the show but nothing is perfect, is some one will be tasked with making an alien or monster and what they come up with seems to have no relationship to any conceivable environment. Evolution trims away the superfluous, go back to the T-Rex and the stubby arms, and keeps the useful. A pragmatic approach to survival in an ecological niche is, in my opinion, the best place to start when designing a monster. Undead are a notable exception to this as they operate under rules that may step outside the natural environment. Then again the Draugar in the Fate of the Norns cannon are fairly straight forward as they are corpses possessed by a spirit and juiced up with extra power.
Until next time.