If you're talking about modded Subnautica, the huge thing is probably the Gargantuan Leviathan.

The one from whatlike ghost or gargantuan or whatever run

You might be talking about Ghost Leviathans, you can find them on this wiki

Nope, it must be modded, that feels kms long

Is it a skeleton. If yes then how big is it. if its head is bigger then your cyclops then it is the Gargantuan leviathan which isn't modded

No, it is moving

What biome is it in

I don remember, pretty empty and dark, the mob roared, icy i belive

There is no ice biome in Subnautica 1

Probably lost river ig