Slaken were huge reptilian-humanoids that inhabited the planet Lumana along with the Loomish. They were huge, green, reptilian cave-dwellers that exhaled fire. The males of this species were maternal by nature, and at least one adopted a group of the child-like Loomish.

When the Tenth Doctor and Martha visited a jungle on Lumana, they were conned by the Loomish into believing that they needed protection from the Slaken which wanted to eat them. (COMIC: House Pests)