Not being a dog per se, but my biggest fantasy is having a bulb in my penis. And fucking a girl with it doggystyle, short slow strokes at first, then when shes good n opened up I'll suddenly let loose and pound it all in so she doesn't know what's going to happen until its already too late to back out. I wrap my arms around her waist and belly and squeeze so hard she can only breathe in little huffs, pulling her against me so tight that she'll have bruises on her ass as I'm swelling up inside her body hard n fast, and shes in total fucking shock. Of course, this being a fantasy shes too turned on to be terrified. She's not quite sure whats happening but does understand she's being made into my own little bitch in a way she never knew was possible, and a fuzzy floaty sensation washes over her from the feeling of being secured so firmly by me. Secured firmly to me. She finally gets enough breath to ask what I'm doing to her and I just tell her it's a weird bloodflow condition with my cock and, oops, I could have sworn I warned her? But now I'm fully swelled up and embedded deep and hard in her body, I let my arms slowly drop to my sides so the muscles can relax without giving me a charley horse because I know she'll stay attached to me for a while yet. She's totally fucking mine now and can't get away even if she wanted to, and there's nothing either of us can do about that but to let nature take its course. I lean over her and tell her this reassuringly while giving her light little kisses and nips on the sides of her neck as I gently rock my pelvis back n forth, her vaginal walls and pubic bone smashed so hard around my knot that her whole body is forced to follow my hip motions while I jet load after thick, ropey load up against her cervix.
Whew! Yeah just one of those funny little fleeting fantasies that might have crossed my mind once.