Knotting is a sub-genre of fic that became popular in the fandom from mid-2010 onwards, originating in one of the fandom's [Anonymous communities and memes|kink memes]]. The knot refers to men having a cock that forms a knot when erect, as is found in dogs and wolves.
The genre overlays canine-type characteristics on people, including pack hierarchies (usually divided into Alphas, Betas and Omegas) and the idea that receptive sex partners may go into heat. Mpreg is not uncommon. There is no single accept set of 'rules' for stories written in the genre, but they usually includes strong themes of dominance and submission. Some stories will build alternate societal and cultural traditions, while others may be pretty similar to real life.