a horny keratinous substance found in two rows of transverse plates which hang down from the upper jaws of baleen whales
But some later lost their teeth, developing plates of bristle-like baleen in their mouths that’s used to filter small prey such as krill from the water. — National Geographic, “Prehistoric Toothless Whale Among Oldest of Its Kind,” 19 Apr. 2018
A lot of Golarion’s (Pathfinder’s Campaign Setting) stories start or involve a costal port, or seafaring society. Even if you don’t stay near the sea, you either start there or visit there it seems. So in their stories, running across a whale, or it’s anatomy is pretty likely.