Hailing from Ogros, a planet in the Tau Ceti system covered in amino acid swamps from which they feed, the Ogri are silicon lifeforms which look like tall stones, much like the megaliths used in stone circles on Earth. They are enormously heavy, weighing over three and a half tons. When they become active - usually when they are fed or searching for food - they begin to glow from the inside. Not terribly quick, the Ogri rumble along the ground while making a loud sound rather like a heartbeat. But instead of leaving a rut in the ground when they move, they leave large indentations rather like footprints; this implies that instead of rolling across the ground, they hop (which is clearly not the case). It may be that they move in increments, settling just long enough to make a depression in the soil before moving on. Whatever the case, their weight is sufficient enough to make crockery and glassware rattle as they approach, akin to the effects of a rumbling earthquake. When they move, they leave behind them a trail of silicon which sometimes accumulates into piles. They also roar when hurt.
Ogri are extremely strong, and can bash their way through stone and metal as easily as a pair of French doors. They are also somewhat servile and respond to commands. On their native world, Ogri feed on the globulin in the swamps, but on Earth, the nearest equivalent is the haemoglobin in blood. If touched by an unsuspecting human, an Ogri can keep their hand attached while it draws out all the blood in their body, which seems to have the side-effect of reducing their victims to skeletons. It is also implied that they can crush the skulls of their victims; presumably they do this by trundling over the individual's head, whether accidentally or on purpose (the hopping theory would provide a neat answer to this conundrum).
Fortunately, Ogri are easily outrun. They can be held at bay with certain laser devices, although they cannot exactly be destroyed short of blasting them into a pile of sand. Allowing Ogri to plunge over cliffs seems to be an efficient way of at least getting them out of one's hair.
