I was a teen at the time, since underage "first time" stories are frowned upon, let's say it was the day after my eighteenth birthday when my first time occurred
One evening I was home alone, and I was a bouncing off the walls horny little fag craving a dick in his ass. Except it was too late for a hookup and the only living creature around was the family dog (a 3yo male Pitbull mix). So, I put 2 & 2 together, and in desperation only teen horniness can bring, said to myself, "Hmmm I want a dick in my ass, the dog has a dick, I wonder if he would be interested in making me his gay bitch?" Curiosity and teenage horniness is a dangerous mix sometimes. I found out several things that night....
#1. Yes a male dog will indeed fuck your ass, quite eagerly in fact if you give him the slightest opportunity.
#2. That little slender "red rocket" thing dogs have gets porn star levels of huge when they're doing the naughty, especially on large dogs like pitbulls.
#3. Once he starts hammering your ass, he ain't going to stop no matter what, so you better be prepared for it.
#4. The knot. Oh Lord God in heaven...the knot!
So, I call the dog into my room, rub his dick through the sheath a bit, assuming that would get him turned on. Then laid down naked across my bean bag chair presenting my teen ass to him. He took a cautious sniff and lick. Seemed a bit surprised I didn't push him away, so he made like he was going to mount me, but quickly hopped down. Saw that still hadn't yelled at him or pushed him away. So apparently said to his doggie self, "Well, ok dude. You asked for it!" He mounted a second time and this time he went for broke, hammering his doggie bone like there was no tomorrow (and in the case of dog anatomy I think it really is a bone, it sure as hell felt like it). First couple times he missed and hammered my balls (which didn't feel very nice). But soon enough he found my asshole...
I didn't know about the knot or dog anatomy at all. But I learned, damn did I learn! Having just lubed up with a small dab of spit expecting that tiny red thing, I felt his dog dick slide in easily, then promptly felt it inflating inside me as it got bigger, and bigger, and then huge! Yes, not realizing what I was doing, I had taken his knot and all. He proceeded to jackhammer my ass like a construction worker on crystal meth, then blew his load in about a minute and decided he had got his rocks off, so was done with me and promptly dismounted and tried to stroll out of the room to go do dog stuff. Except, we were still tightly knotted together, and in a larger dog like a Pitbull, the knot at peak inflation is about the size of a balled up fist. He literally dragged me across my bedroom floor kicking and screaming for about five feet before the knot (very painfully) popped out of my ass, leaving me a wimpering, dog jizz covered mess on the floor. Oh, he made me his bitch all right...
That being said... The ass pounding part was actually pretty damn good, that huge dog dick inside me felt awesome (well until the knot popping out deal) and did the job so well I quickly blew my load, (Lol! much faster than the dog in fact, he lasted a minute, I lasted about 30 seconds). But of course, since I'm here now, I went back for more (taking precautions like plenty of lube). Now that I'm in my 20's I'm a hopeless dog dick addict and have a two-year-old Rottweiler that uses me as his personal cum dumpster.