Bangoons are an alien species that possess the unique ability to turn themselves completely invisible when they are nervous.
For some reason, while they spoke in Season 1 and 2, the Bangoons in Mission: Earth do not speak at all.
Also, the three Bangoons who appeared in Spider Betty cloaked themselves when frightened. X-5 description implies that this is a normal Bangoon trait. However, this trait is not seen in any other Bangoons.