Ogri

The Ogri are a silicon-based species which physically resemble large, luminous mobile stones. They are native to the swamp planet Ogros, in the Tau Ceti System.

Biology
Physically, the Ogri resemble megaliths, irregular in shape, without eyes, mouth or limbs. They can be easily mistaken for stones when immobile. When active, however, they glow with a mysterious luminescence, and are capable of moving at considerable speed despite the absence of visible limbs. While they're only slightly taller than Humans, these massive creatures are estimated to weigh over 3.5 tons.

Despite being silicon-based, Ogri feed on organic proteins, which they absorb through physical contact. The proteins that they consume are similar to the globulin found in Human blood, which can serve as a feasible substitute. In their home world, the Ogri inhabit vast swamps where proteins and amino acids can be naturally found and absorbed by them. However, if they find themselves in an environment where such proteins don't occur naturally, the Ogri will just as readily resort to predation. Their hunting method consists of approaching organic life forms and remaining immobile, camouflaged as common stones. When the unsuspecting prey touches the seemingly harmless Ogri, the prey's flesh is absorbed at remarkable speed, leaving only the bones behind. Alternatively, the Ogri can survive by consuming Human blood directly, provided that another creature collects it for them and feeds them. They can't feed on the blood of Diplosians due to differences in the Diplosian metabolism.

Ogri can live for several millennia. Their sapience level is not clear, although they are at least intelligent enough to understand verbal commands. They don't seem to be capable of speech.

History
Circa 2,000 BC, a Diplosian criminal known as Cessair of Diplos fled to Earth to escape the Megara, and took three Ogri with her to serve as bodyguards and enforcers. She landed in Boscombe Moor in Cornwall, England, and remained there for nearly four millennia, being worshiped as a goddess by generations of superstitious followers. To keep her Ogri servants alive, Cessair had her followers perform sacrifices and collect the blood of animals to feed to the Ogri, which the Humans mistakenly regarded as stones.

By the time Cessair was finally apprehended by the Megara in 1978, with the help of the Doctor and Romana, two of the Ogri had been killed, and the surviving one was kept by the Megara within their ship, as they planned to return it to its home planet.

Appearances

 * Doctor Who, season 16, "The Stones of Blood" (1978)