Bulbus glandis

The bulbus glandis (also called a knot) is an erectile tissue structure on the penis of canid mammals. During mating the tissues swell up and lock (tie) immediately after penetration of the male's penis inside the female. The locking is completed by circular muscles just inside the female's vagina tightening thus preventing the male from withdrawing. The circular muscles also contract intermittently, which has the effect of stimulating ejaculation of sperm, followed by prostatic fluid, as well as maintaining the swelling of the penis and therefore the tie, for some time. For domestic dogs the tie may last up to half an hour or more, though usually less.

The bulbus glandis also occurs in the penises of some pinnipeds, including South American fur seals.

...
Wikipedia