The Ourang-Outang isn't particularly a character, but neither is Dupin, so labeling him an antagonist is acceptable. On the plot level, Dupin tries to solve a particular case to get Le Bon released from prison. But what he's really going up against is the idea that anything should be unsolvable. Remember the epigraph, in which Sir Thomas Browne tells us that even the deepest mysteries should not be beyond conjecture (check out "What's Up with the Epigraph?" for more)? Well, Dupin is living proof of that: he takes a case that has no motive, in which the antagonist is not human, and manages to solve it anyway. The Ourang-Outang is a living symbol of irrationality, and Dupin succeeds in overcoming both the literal ape and the emotion and physical strength that he represents.