Emohawks are a form of shape-shifting polymorph.
Unlike the "space monster"-like natural appearance of the polymorph, the natural form of the emohawk is something similar to a large bird or hawk. It also makes sounds like a bird. However, like all polymorphs, the emohawk is an emotional leech - it steals emotions from other living creatures. Its name is a portmanteau of "emotion" and "hawk."
Emohawks are domesticated variants of polymorphs that are spayed at birth, raised by the Kinitawowi and trained to do their bidding. Emotions drained by the emohawks are a highly valued trading commodity among the Kinitawowi.
The emohawk seems to be able to drain specific aspects of an individual's personality as well as mere emotions, and causes its victims to undergo physical changes in addition to mental ones; it drains Arnold Rimmer of his bitterness and Cat of his cool, turning them into Ace Rimmer and Duane Dibbley.
Unfortunately, the Chief demanded that Dave Lister marry his eldest daughter. Lister went through with the ceremony, but before the marriage could be consummated, Lister ran away back to Starbug. His new bride was distraught, and the Chief sent an emohawk after the Dwarfers in retaliation. It sneaked aboard Starbug in the form of Lister's hat.
After causing much chaos among the Dwarfers, including turning Arnold Rimmer into Ace Rimmer and Cat into Duane Dibbley, the emohawk was defeated when it was frozen with liquid dilinium. Unfortunately, Duane Dibbley also accidentally froze his crewmates with it. ("Emohawk: Polymorph II", Series VI)
When the emohawk didn't return and the Kinitawowi realised that the emokawk had failed, they instead chased the Dwarfers in a commandeered battle cruiser across space to demand Lister's return to his wife. ("Ouroboros", Series VII)