Unponies, or Almost-Ponies, are faulty clones ruined by their exposure to amniomorphic gel. They were created during the apocalypse when a magic-induced earthquake struck GivingHoof Labs, causing the aforementioned gel to spill into prototype cloning vats.
They retain some degree of sentience, though their bodies are wildly mutating piles of unstable chemical bonds. They most often appear as semi-translucent, gelatinous pony-shaped blobs with a host of strange appendages, though their very nature gives them no uniform appearance.
Both Lozenge and Wireless note that some Unponies are not aggressive, while others are mindless predators. A few even retain rudimentary linguistic ability. Mother Unpony had even evolved a means of psychic communication.
-Source: Los Pegasus Confidential
-Source: Mad Circuitry