Cronenbergs are genetically-mutated creatures made accidentally by Rick in the episode Rick Potion No. 9 when he tried to turn the "Mantis-people" back into regular Humans. Everyone on Earth became cronenbergs except for Jerry, Beth, and Summer. Despite being a side effect from the antidote, there is a whole universe of Cronenbergs which was "cured" by Cronenberg Rick and Cronenberg Morty.
The word/name Cronenberg is a reference to Canadian filmmaker, screenwriter, actor, and director David Cronenberg. This is due to the fact that Cronenberg is one of the originators of the "body horror" genre in which the style of filmmaking explores people's fears of bodily transformation and infection.