It’s called a surgeonfish because it has at least one spine that has evolved to be as sharp as a surgeon’s scalpel. These spines are near the root of the tail and fold into the body in many species. You can find where they are because the area is usually a different color than the rest of the fish. For example, the Sohal surgeonfish has blue and white stripes but the area where it keeps its “tailthorn” is orange.