Ok but there is a hole in the bulb zone/ mountains that leads straight to the lava zone where sea dragon leviathans are. Note when reading the pda entry for the reaper skeletal remains it say “ it was likely dragged down here and boiled alive.” Also note that the hole leads to the lost river with ghost leviathans where they might be hunted. Plus there is a sea dragon skeleton so they might not be afraid to go to the lost river.

Well, I am out of ideas. My last idea is being bigger than some creature on earth. Simple. I hope there will be explanations in Subnautica 2

Levithan is a large sea serpent from the Bible. So I guess what makes a levithan a leviathan is it's size?

Largest Apex predators in a given area? The reapers do get eaten by the Sea dragosn, however, I dont think the sea dragons are a natural predator of the Reapers. SO the reaper leviathan is an apex predator. Ghost leviathans aren't predators, right? I remember it being mentioned somewhere they indulge in cannibalism, I don't know if that's just in my head though.

Leviathans do not just have to be predators because there's reefbacks, ventgardens, sea treaders, and glow whales, which I believe are all herbivores and leviathans.

True true.

I did some reasearch and theres no other description other than large, monstrous beast, which resides underwater

What about the ice worm

Ice is frozen water

But ice in large amounts is land not sea

Don’t the PDA entries state if they are leviathans?