Howler monkeys have few, if any, real predators. Humans may sometimes kill a howler monkey for bushmeat, but otherwise their main threat is the harpy eagle.
Like the New World howler monkey, the harpy eagle lives in a range stretching from the the southern half of the central Americans, down through the northern half of South America.
Harpy eagles have powerful talons around three inches long, and are quite capable of snatching a howler monkey. The howler monkey is defended from most potential predators by its preference for living in treetops and forest canopies, but harpy eagles specialize in catching prey in such environments.
This said, the harpy eagle is a carnivore and so will attack any sort of animal smaller than howler monkeys, who can put up a good fight against a harpy eagle if needs be. Doubtless an eagle would prefer easier prey for its meals than howler monkeys.
Humans also pose a threat to howler monkeys in terms of depriving them of forests in which to live, or by capturing younger monkeys to keep as pets.
Learn more about how eagles can prey on howler monkeys by watching this BBC film: