The Kreel are a space-faring humanoid race of primarily scavengers. They have frequently come into conflict with the Klingons since the 23rd century, due to their habit of picking at the remnants of Klingon defeats or victories after the Klingons have departed; they have been compared to jackals.
The Kreel physique consists of broad, heavily muscled (almost triangular) torsos and long arms (hanging nearly to their knees) with three-fingered hands, supported by incongruously spindly legs. They display little or no neck structure, limiting the motion of their heads and requiring them to physically twist around to see to their sides or rear. The have huge eyes compared to most humanoids (ca. 7-8 cm in diameter) beneath an overhanging brow ridge, and commonly also have a "lantern jaw". Their skin is extremely wrinkled and dry, "sunburned red" in color, and their bodies have a thin layer of coarse, matted hair, though the thickness can vary over various parts of the body. Kreel clothing tends to be simple and minimal, usually breeches and skimpy tunics, to better show off their physiques. (TNG novel: Strike Zone)
In early 2365, the Kreel discovered a stash of advanced weaponry on an uninhabited planet and began attacking Klingon ships with them. The USS Enterprise was called in to mediate the dispute. (TNG novel: Strike Zone)
In 2376, the Kreel assisted the Al'Hmatti freedom fighters and attacked Klingon ships associated with taD. The IKS Gorkon was called in to escort Ambassador Worf to mediate the dispute. (TNG novel: Diplomatic Implausibility)
During 2378, the Kreel were one of several races to attempt to scavenge the Rashanar Battle Site. (TNG novel: A Time to Be Born) Wesley Crusher had disguised himself as a Kreel in order to rescue Jean-Luc Picard. (TNG novel: A Time to Die)
At some point between 2377 and 2380, the Kinshaya conquered the Kreel and absorbed their resources as the first step of their escalation in their long-time conflict with the Klingon Empire. (TNG novel: Q & A)