When I was on like level 20 in single player i used to find these guys everywhere, and i thought they were only useful for a handful of chitin. Now I’m on level 88 and i actually NEED black and silicia pearls and oil I haven’t seen one in months.
It all depends, sometimes you’ll find a bunch of them, sometimes you won’t find any. They spawn very frequently near large masses of shallow water— but the water must connect to the ocean. I find them in the mouth of the rivers near shallow water. More often then not they spawn in the south or near the red obelisk if you pace yourself towards the snow biome you will definitely encounter a few along the beach.
Fun fact: The trilobites appeared during the Cambrian explosion and was one of the most successful lifeforms on the planet. Nature had to try and wipe them out about 3 times and it took the most deadly mass extinction in the history of our planet to wipe them all out. The Permian Extinction also known as "The Great Dying"