Jorge sat down on his rickety old porch just outside of Mexico City at 2:37 AM. He pulled out a cigarette and a match as he turned his radio on. He struck the match and started pulling it to his cigarette when he heard a noise from his pasture in the back. Just his sheep getting restless. He resumed the process when he heard the sheep equivalent of a blood-curdling scream. Now, that couldn't be ignored. He took the lit match to the pasture and started an investigation, he was trudging along the fence-line when he saw something: flattened grass from a creature, roughly the weight of a big dog, it seemed. He followed a series of flattened patches of grass, like a hopping creature. He finally heard a sucking sound as his match light fell on... something. It was the size of a kangaroo, but it was pale green and had leathery wings. It stopped mid drink, for it was sucking on the sheep's neck, and whirled on the man. It screeched and leaped at him.

The Strallen is a creature from Menisco, which is a small chunk of sulfur surrounded by thick gases similar to a gas giant. Their leathery wings catch the gas currents and ride them around the planet, feasting on prey. One sneaked into the air vents of a low-flying spaceship and couldn't get out. Instead of taking him back home, the aliens dropped him off on the nearest livable area: a few hours outside of Mexico City, the most polluted city on Earth.

Unfortunately, his wings don't work in Earth's thin air, so he has to hop. Also, the atmosphere is just barely enough for him to breath, so he's constantly wheezing, which is often mistaken for a screech. Also, his hunting of local prey has given him a new name: El Chupacabra.

This is based on an idea I had for a Doctor Who episode, so please enjoy.