I'm currently developing a military sci-fi concept and could use help working through some theoretical creature designs.
During early concepting, the actual specifics of the creature-threat took a back seat to the human drama at the centre of the story -- the creatures only needing to serve as a generic "threat" to steer our characters towards specific choices and shape our storytelling goals.
However, now that those goals have been identified and we must start working step-by-step through the actual narrative, the specifics of the creatures themselves takes first priority. Thus far, we've been thinking in broad strokes about how it might be interesting to have a hostile species that hunts by vibration and has trouble locating someone standing still.
A placeholder I've been working with in my head is something like the Central American Wandering Spider and how they respond to vibrations within specific frequencies and amplitudes, like the movement of prey on local foliage. This is almost exactly what I'm looking for, but wonder about the impact of scale. I know that you can't scale an organism up to any significant degree without requisite changes in its structure due to the square cube law (and from what I think I understand of Kleiber's Law), so I can't just say that this hostile species is just a "big Wandering Spider." It also faces the challenge of detecting vibrations on something more stable than a web or foliage -- grass, concrete, packed dirt etc.
In summary, the basic characteristics I've been working with so far:
Large Size: If it hypothetically had the proportions of a Daddy Longlegs then the body would be hovering somewhere around your head and maybe the size of an armchair.
Fast, Agile: Able to exhibit an incredibly fast attack response. Jumping?
Able to sense gross movements like someone running, walking -- perhaps not tip-toeing, at least not from afar.
Probably poor eyesight if not blind.
And yes, I do know I'm basically cherrypicking characteristics from different body-types like a kid running through a supermarket filling up the cart with every candy and cereal-box that catches his eye without any heed to the concept of money.
I could just write it off as 'Rule of Cool' and proceed with execution, but I don't want to go the Godzilla or 8 Legged Freaks route and pretend that there's no such thing as science. This isn't a fantasy or science fantasy; I do want to keep the sci-fi as "hard" as possible (within reason, of course.)
Thus, if anyone more educated in the relevant science would be willing to simply think aloud and riff on the subject of such a fictional species, know it would be greatly appreciated and help me in the creation of a (hopefully) compelling story that seeks to not disregard all science in pursuit of storytelling ease.