I've seen them more often outside of Thirsty River in the desert. They look like little stationary lumps, until they "wake-up". Then, they're kinda like a stationary tentacle.
They look like mini siege worms. They are a little bit taller than your character, but nowhere near as large as a siege worm. I saw one near the collector for the jaws. It was in Arrid Sea.
I might add. Be carefull when going to fight them.
they usually have scarabs near them. you wont see
the scarabs, but dont worry they'll come out of the
groud on time to greet you.