By this I mean creatures that just seem to float around, seemingly under no power of their own. Things that move like this tend to be very hard to escape from, as little will really stop them. They're also silent.
The creatures on Earth that do this are mostly cnidarian animals like siphonophores or jellyfish. I absolutely will not go in the water if there are any jellyfish around. You simply can't reason with it or get into its head in terms of guessing what it will do next. That's the real scary part. Just what is it thinking about, if anything?
The classic Dungeons and Dragons Beholder floats around eerily.
The Portuguese man o' war is a siphonophore that aimlessly floats around.
“I will not go in the water if there are any jellyfish around”
Some cubozoa like the box jellyfish have been known to grow eyes.
The classic flying saucer has no visible method of propulsion.
A floating human nervous system would be absolutely terrifying.
The brain from planet Arous is essentially a floating human nervous system.