Ship-scale creatures?

[Galadrion]

I'm using marine creatures for a lot of the inspiration.

That makes sense, in that it's one of two example biomes from Terra which is (primarily) three-dimensional rather than two-dimensional - the other being aerial creatures (birds, bats, insects, "ballooning" life forms such as pollen/spores and ballooning spiders). And there are a lot of bioengineering overlaps between the two biomes, but we really understand the marines ones (forgive the inevitable pun) in more depth. Still, if you want examples geared more towards speed, perhaps you should consider looking at some of the birds, especially.

 

[Anstett]

Would speed matter? Given the idea that the original 'soup' is a gas giant, then the lines of an evolutionary path could be:

Surviving the radiation etc of space would be the priority,
then Movement to move between "small rocky balls" (moons)
then Endurance to move between larger rocks and advancing into moving between systems
Only after a species has propagated out far enough among the stars would Speed come into play.

The ability to chase after a mate, or avoid conflict.

 

[Fluffy Bunny Feet]

The larval Krill obtains its energy in the same manner as the adults relying on the weak gravity of its home to attract the gases it requires to live.

Alternately their origin could have been in a "Smoke Ring" as in the Niven novels "The Integral Trees" and "The Smoke Ring'.

Now if you want REALLY big creatures of great power read Roger MacBride Allens "The Hunted Earth" books.

 

[GypsyComet]

TNG also had the creatures from the first episode that mimiced the Starbase and the "baby" that leached energy from the Enterprise. Then there were the beings that wanted to kill Q when he was mortal.

I mentioned the Farpoint Station beings earlier. Trek has a lot of examples.