A "hybrid" in the world of plant and animal biology (poultry, too) is the cross of two distinctly different breeds.....a MUTT is essentially a hybrid by the actual defintion of the word! The best example of "hybrids" today are those high dollar designer MUTTS at the pet stores! LabraDoodles, which are a cross between a Poodle and Labrador Retriever. Anything that is crossbred is essentially a "hydrid".
Now in the world of chickens (both egg and meat) there are "Hybrid" varieties but we don't generally call them breeds. The most common in the egg layer poultry are Golden Comets, Black Stars, Red Stars or other "sex-link" hybrids that are created by crossing two different chicken breeds. In the meat chicken world there is the Cornish Rock or Cornish Cross (CornishX) which is generally a cross between a White Cornish and a White Plymouth Rock to create a fast growing hybrid. The commercial meat chicken industry has gone even deeper by combining multiple crosses to create 4 way "hybrids" but the bottom line is they are NOT a breed they are a cross or "hybrid" in general terms!