Rot is a lab experiment. He is composed of thick oil, and a strange bubbling and green ooze. The ooze around his body form a sort of 'rib cage' and a large pouch as a 'stomach'. The 'stomach' is lined with a thick membrane to capture prey and is cable of keeping prey that are almost 10x larger than he is.
The green ooze inside of him is a very strong acid, in which can suffocate prey fairly quickly and dissolve them into a mass of indigestible parts within a hour or 2, depending on how large the prey is. Rot only absorbs the necessary nutrients ( i.e. flesh, blood, & organs ).
On another note, besides him being a huge predator, he's actually very sympathetic and shy towards close friends. He usually makes friends quickly ( despite his appearance ) and only eats things as a defense mechanism.