This is a large humanoid inhabitant of the planet Mars which, in 1973, killed a number of astronauts in an exploratory mission to the red planet, subsequently hijacking the spaceship which had been sent to rescue the sole survivor.
The creature is a muscular upright biped, strong enough to easily bend a riffle and even tear down a metallic wall. Its body is covered in scales and has a flat nose like that of a bat, long teeth, and three clawed digits on each hand and plantigrade feet.
It also shows intelligent behavior in operating some of the ship's mechanisms, leading the crew to speculate that the creature belonged to a once-sapient and civilized species which degraded into barbarity as the planet's ecosystem began to die due to loss of atmosphere and water bodies. Accordingly, the Martian has a bizarre way of feeding, in which it drains out every single molecules of water and oxygen from its victims' tissues, leaving a completely dry carcass behind. They further contemplate that the entire modern ecology of Mars might operate on basis of larger creatures stealing body water and oxygen from smaller ones, which in turn take it from the atmosphere and soil.
The Martian is a remarkably resilient creature, seemingly impervious to bullets and able to withstand grenade explosions and electrical discharges strong enough to kill 30 Humans. It is however highly vulnerable to the absence of oxygen, and needs to consume huge amounts of the gas when not able to obtain it from predation.