Slangily called a "dog knot", the male structure is a bulbous globe of erectile tissue near the end of the dog's penis, and it engorges and precludes (or makes difficult) the withdrawal of the penis immediately after ejaculation.
I never got as far as ejaculation, so I wouldn't know for sure.

AAOS: What about some real experience first with animals. (I do not mean beastiality, just that you learn a few anatomy: birds have wings...)
More concern about the claw might also be welcomed.
Yeah, there's another concern...one I didn't have to deal with, fortunately.