The term "gregarious" refers to the tendency of animals to be sociable, living in groups or communities. The sentence states that gregarious animals can be found in the zoo. This implies that these animals are comfortable being around others of their kind. The opposite of "gregarious" is "lonely." If an animal is described as lonely, it means that the animal is isolated or lacks companionship, which contrasts with the sociable and group-oriented nature implied by "gregarious." Therefore, "lonely" is the most suitable opposite in this context.