Go the Silurians route and fix it up later.
I want to say go with creatures that would logically appear in the habitat you build but that could get mundane and that's the last thing you want for your creatures. Levellass would say design the creature, think of a behavior for it and then patch it to match it (your vision). A beginner like me has to go about it the other way around. The best sprite I think I ever drew was a snake for the Keen1 robot tank replacement and that wasn't even entirely original. I tried to draw up a lank shark as a replacement for the Wormouth. It was based on an enemy fish from Kirby Amazing Mirror and the shark from Wacky Wheels. My very first attempted at an enemy was making a lizardman form Hocus Pocus. It was then I learned the bitmaps for the sprites must be a certain length divisible by 8 or some such thing. Start out simple with the patching. Stick close to the Krodacia style. Steal a complex patch or two from Terror From Outer Space. Experiment with animation speeds. You can make creatures move super slow or blindingly fast. Simply changing one value with a little patch can have a satisfying impact for sprite replacement.
Maybe make the mimrock jump like crazy, replace the art with a shrunken dopefish, sharpen the teeth, give it a mean attitude and spawn it down in the dangerous lava waters. If it moves, it must have feet, tread, springs, propulsion. Maybe disguise an enemy. Creatures are made to stand out. Maybe make one that blends in with the tiles. Maybe make a lethal airboard/goplat imposter. Now getting the 2.5 perspective right, making the movement smooth.... oh dear, that
is a lot of work.
This looks pretty cool, and rather unexpected! It's always nice when people come out of nowhere and show us that they're working on a mod! 
The Atroxian Realm influence is pretty strong here. I really like how the background in the second screenshot looks! Looking forward to see how this goes!
