I've had a few ideas for creatures to make traveling in space and on other planets more dynamic; unfortunately I have no ability to code them, nor do I have the time to learn how, despite my interest. Here are a few ideas for your consideration:

Gossamers: Gigantic jellyfish looking creatures that are largely passive and live in space. They are attracted to your platforms thruster emissions and will pursue and attack your platforms while in transit. If found near a planet, they will not attack unless provoked or your platforms thrusters turn on. They are completely immune to electric and fire damage, nearly invulnerable to physical damage, but are nearly defenseless against acid and poison damage. I'm not sure if it's possible, but an advanced offensive capability would be communication jamming, preventing you from controlling your platform while it is attacking. Their healing factor is 1/8 that of demolishers, but have twice the hit points.

Reavers: Native to Vulcanus, these crab like biters are somewhat slow, heavily armored biters where the behemoth versions act as mobile spawners (again, not sure if possible). Nomadic and usually found in groups of up to 30, extras that spawn in the group will break off and form a new group, actively avoiding other groups as well as (and especially) Demolishers. They are also incredibly aggressive and will attack on sight, just like biters. However, they will send a call for help when attacked; aggroing every available group within 2 chunks. They possess the same healing factor and resistances as Demolishers, but with HP similar to that of Biters.

Scarubs: Found on Fulgora, this is a very strange creature that is completely immune to lightning damage, only moderately resistant to physical damage, and killing it with physical damage may actually split it (50% chance); respawning it along with a second one as well. They are also very small, with the behemoth versions only as big as a medium biter. Despite their resistance to physical damage, fire and acid damage seem to be fair game.

If you like any of those ideas, let me know how possible they are.

Thanks a lot for taking the time to write all this up, there are some cool ideas in there! I already have my own plans for the next creatures, so I'll be focusing on those first. That said, with your permission I'd like to keep your ideas in mind for later on, once the ones I have planned are in, and I'll credit you if any of them make it into the mod. Thanks again!

Absolutely, I wrote this so that you could do something with my ideas as I cannot do so myself. So, yes you may take these ideas and see what is possible.