These creatures I've made live on an almost entirely oceanic planet, a water world with harsh mineral seas and a savage ecology. I got the idea from coastal spiders and flora I remember seeing as a child, which, because of the strong saline environment, have evolved hard, spiny, tough forms out of necessity for protection from the salts and minerals. Similarly, there are in the sea many creatures which have land animal equivalents. For example, crabs and crustaceans are like the insects of the ocean, skates and rays are like birds, sharks are like predatory canines. It's not hard to begin imagining from there what else there might be.