Knot
Knot
The knot (also known as bulbus glandis) is a structure at the base of a dog’s penis which enlarges during intercourse, effectively attaching him to his partner in what is known as a ‘tie’.
Due to the baculum, a dog is able to penetrate before his penis is fully hard so the knot slips in easily before engorging with blood. Once the knot has formed the dog is considered fully aroused and at this point he will be cumming, little spurts that shoot out intermittently for the entire time he remains hard.
Oftentimes people want to know “What is the size of a dog’s knot?” The correct answer: |The size of a fruit.”
Some of you may be wondering now, “Well then, what fruit exactly?” To which we can only reply, “That depends on which dog, exactly!”
In general, small breeds tend to have smaller penises with smaller knots, while large breeds usually have larger ones. However this is not some sliding scale, expected to be always accurate. There are some large dogs (especially sighthounds and fine-boned breeds) which have small genitals proportionally compared with their weight and/or size. There are also some very small dogs (Chihuahuas, Beagles) which may sometimes be gifted with a larger penis than expected (and likewise the females with rather elastic vaginas). The fact is, one will never truly know what to expect until taking a look at that particular canine.
References
http://beastiality.club/beastiality-club-extreme-animal-sex-content/the-encyclopedia-of-zoophilia/