The Skrak were a species living in the Dagellan Cluster. They were short, furry, had three eyes and spoke Skrak. When they brought the Seventh Doctor to the Cluster, they were masquerading as a semi-intelligent race on the planet Moriel called the Plob.
The Doctor speculated that they had only been in existence for twenty years. Their parasitic relationship with the Otherlings reminded him of the symbiosis of extradimensional creatures and mammalian rat-vermin caused by the fallout from early Gallifreyan time experiments. Their lives were in a sort of time loop, eventually driving them insane.
The Skrak created the Hollow Gods as part of a convoluted plan to take over the Dagellan Cluster, then the universe. Their leader, the Mighty Leader of All Skrak, was killed by an out-of-control automata. Leaderless, the Skrak were thrown into confusion and imprisoned by the Czhan, Dakhaari and Saloi. They managed to escape, steal a spaceship and leave for parts unknown. (PROSE: Death and Diplomacy)