The Last Martian was a cephalopod and ostensibly the last survivor of the oldest indigenous sentient species on Mars. In reality, it was bioengineered by Paul Ares and the Ares Corporation so that the Ice Warriors would not claim Mars for themselves.
The Last Martian killed Ares when it learned he meant to kill it and replace it. The Last Martian was killed by Leela during its attack of the Fourth Doctor's boat Dejah Thoris. (PROSE: Crimson Dawn)