The red-eyed creature was an unknown alien lifeform that was accidentally brought to Earth.
The creature had two red eyes and was about the size of a small human. It could recover swiftly from its wounds.
The creature was brought to Earth by the experiments of Erasmus Darkening. It took to prowling the grounds of Ashen Hill Manor. It tried to kill Sarah Jane Smith and was in turn wounded by James Marchwood. When Darkening's machine was switched off, the creature was presumably returned to its rightful place. (TV: The Eternity Trap)