The syndromes caused by forgotten beasts are random. One of the funniest involved the Bay12 games fort Battlefailed, one of the forgotten beasts they dealt with spread a white powder everywhere that caused numbing on contact. There were jokes about cocaine, but then, several years later, every dwarf who'd stepped on it barefoot started having their feet rot off. The necrosis effect took a very long time to actually show up in-game.