Each swamp monster consisted of a large, gelatinous mass that housed the brain and other vital organs. They also had a number of large tentacles protruding from the central mass. The brain and nervous system were spread across the body, meaning that their ability to think was slow. They kept their bodies submerged in water because they were so large and interacted with dry land using their tentacles. Each tentacle could penetrate the skull of another being; this allowed it to control them in a similar fashion to a puppet. By attaching tentacles to other beings, a swamp monster was capable of utilising their skills and intelligence.
Their lack of intelligence meant that they acted mostly on instinct to survive. As mentioned, they can use other species to help achieve their goals. For example, the Swamp Monster on Sunday utilised humans to communicate with others. It also used a protein-RNA mix to control smaller animals, such as the Sundayan otters; this mix implanted memories and images into them and generally led to a rise in aggression.
They believed that reproduction was the purpose of life; they achieve this by blowing their bodies up. A powerful enough explosion could project each piece of swamp monster into space, where they can spread to other planets, in a similar fashion to stage trees, Krynoids or the comet flowers on Besseme. (PROSE: Wetworld)
Sometime after the arrival of the Sundayan colonists in 2107, a small piece of swamp monster landed on the planet Sunday inside a meteorite that struck the planet. It took time to establish itself; the colonists first noticed that something was different when the local otters became more aggressive. After the arrival of The Tenth Doctor and Martha Jones, the creature began to attack the colony. It killed and took control of Pallister the head of the Sundayan Council and former nuclear technician, intending to use his knowledge of nuclear technology to build a nuclear weapon and blast itself into space and spread to new worlds. The Doctor managed to defeat it by allowing it to begin to take control of him; he then injected the creature with a new protein-RNA mix, designed to destroy its outer membrane. The creature died shortly after. (PROSE: Wetworld)