It's a thing. An artifact from bestiality.

Canidae is the family of carnivorous and omnivorous mammals commonly known as canines. It includes dogs, wolves, foxes, coyotes, and jackals. These animals are all digitigrades, meaning they walk on their toes.

When mating, male canines develop a distinctive knot (the bulbis glandis) at the base of the erect penis which tends to trap the phallus inside the female's vagina for some time after copulation, ensuring greater reproductive success

You can buy dildos of it. Uh, NSFW, of course.

Without question this is the closest experience to the real thing that you can have.

Oh, yeah? Will it give me fleas?

Jeez, I just had to ask . . .

Thankfully I've managed not to stumble across that particular kink.

um ... I'm almost ashamed to admit it (almost) but I've run into some books where men have what they call "the barb" - same thing. ow

It's nice because you know there will be cuddling...

I clicked on ita's link. sigh ... it amused me that it's molded from a Labrador Retriever. Poor dog!

I suspected I knew what 'tying' would be about but I did not expect the dildoes.

No one expects those dildos.

Their chief weapon is surprise... and fear.

Anyway ... this is something I've seen in werewolf AUs, but I'm stumbling across it in fic where some human males are "alphas" and knotting is part of sex. Has anyone seen this in other fandoms?

I must have, because I knew the answer. But it was so long ago I can't cite my source.

edit: but I'm guessing it was an alt.sex newsgroup.