A Phyrexian sleeper agent is an organism grown from harvested flesh stored within the Phyrexian flesh vats. The original Sleepers had no gender, were short (about the height of a child) and did not age. Called "newts" they were some of the most expendable forces ever created on Phyrexia. Many of the first newts were slain upon arrival to the planes they were to inhabit due to the differences which made them obvious to those they met.
Later sleeper agents had a gender, naturally aged, and were usually unaware of their Phyrexian origin until the time they would be used for various chores, including the construction of portals to allow Phyrexians to enter other planes.