They grew up to two metres long. Humans thought them monstrous because of their violent behaviour, but they were highly intelligent; they could build a highly technological stronghold. They also thought of humans as animals, causing Rose to realise both races had misconceptions about each other.
The Mantodeans were horrified when a Quevvil-controlled Rose Tyler shut down all of their power, but they were so angered by the Quevvils and busy plotting their revenge against them that they didn't notice that Rose had escaped. They were also unaware that the Ninth Doctor had defeated the Quevvils with the help of Mickey Smith. (PROSE: Winner Takes All)