A powerful and corrupted nymph could be the main antagonist. She was once a friend to the Hunt but was lured to the dark side by some ancient magic. Now she wants to destroy the Hunt and take over their territory. She uses her powers over nature to create all sorts of obstacles for the Hunt and Percy, if he's involved. She could also be very manipulative, turning some of the Hunt members against each other.
An ancient Titan who was imprisoned long ago but has now escaped. This Titan sees the Hunt as a threat to his plans of ruling over the demigod world again. He has vast powers and knowledge, and he uses his minions to try and take down the Hunt one by one. He might also try to corrupt Percy if he thinks Percy can be turned against the Hunt and used to his advantage.