I had really issues finding them since I had no idea how they looked like, so here is also a picture oft them. They are huge (compared to other similar crabs). Picture of size (compared to my archer) is linked here: https://imgur.com/AUSvKKY Keep inmind that they do not appear on your map as red dots.
You can find them @ the screenshot Anya provided, they'll be on the road looking like a big rocks. They have specific shape to them so it's easier to spot them. You can check @Ctulhus_Requiem's screenshot to see what they look like. It took me a while to find them until I checked the screenshot and they are just out in the open.