Alpha/Beta/Omegaverse ... basically someone in some fandom somewhere came up with an AU verse where men are alphas or omegas and the omegas have man-ovaries and go into heat and require an alpha to do 'em, and they're very weepy and want only babeez! And the alphas are very butch and take them some omegas and there's knotting and marathon sex and the omegas have the babeez outta their butts. Oh, and there's soulbonding and all sorts of id-tastic things.

It caught on and now nearly every fandom has A/B/O fics, and some have LOTS AND LOTS of A/B/O fics. The Hetalia kinkmeme is chock-full of Omegaverse prompts. (I've never seen one in Saiyuki, however,)

I did an Alpha/Omega fic but with no mpreg and basically did it only for the dubcon and marathon sex, thus I was doin' it wrong. :)