This is speaking of canid
animals (members of the dog family; Canidae). A knot
is basically a swelling of the Bulbus Glandis, a part of a canine's penis. The erectile tissue swells up during the climax
of sexual intercourse and locks when it penetrates the female allowing it to inseminate
into the female canine (probably because these canid animals miss when they hump the female so the semen will miss, I think).
Dirty Friend: "Lol. Look at that dog's knot. Now it's stuck with the female"
Other Friend: "You're disgusting. You're probably thinking about bestiality you furry freak."