dogs don't get an erection prior to penetration (that's what the bone inside is for) and a soft canine penis is very slim, calling it "pencil-size" is accurate for most breeds, "smaller carrot" may be more accurate for the truly well-endowed ones.
what happens? they tie, the knot expands and stays in... then goes out when it's soft again. granted, nothing unusual (what i mentioned in my first reply) happens... doing "stretching training" to get looser or whatever is what can lead to a painful pullout, because the knot has a way to unintentionally slip out if you're loose enough. the boy will pull during a typical tie, pulling on their penis makes them orgasm harder.
ask literally any woman how hard is it for them to hold vaginal tie just because a vagina is looser than an anus by default.
purely anecdotal on my part (i bottomed for multiple boys so far), but i don't think there exists a knot large enough to get past "slight discomfort" when anally tied. what can happen is him not penetrating deep enough and his knot starting to swell in the anus, which you will definitely feel (pretty sharp pain) and can easily pull out to let him try again.
might be just me, but i believe that a properly done "goes in soft, goes out soft" canine sex is way safer chance of injury-wise than letting a human or a dildo in. both go in at full size, a dog doesn't.
various canine penises are all that ever entered me (not even a finger and i intend to keep it that way) and i never had any injury. well, a serious one.. an inexperienced boy can stab you on the rim with the bone which can draw some blood if he's eager enough. i highly doubt i happen to have some magical "no training needed" butt...