My sister had a horny male dog that I didn't think of in a sexual way at all, but I was in her room and down on all fours (for another reason) and was crossdressing her clothes including a short skirt and thong panties and because of the position and clothing I was wearing giving him access, he mounted me and was able to get inside my asshole and started fucking me hard. I was in shock at what was happening and didn't more right away and then it started feeling good, so I let him keep going not knowing anything about dog sex or about male dogs having a knot on their cock. When he stopped, he wasn't finished like I first thought. I didn't know why we were stuck together after he stepped off of my back and turned and why I couldn't get him out of me, but later found the reason was because he had knotted with me. My sister came home early that day and caught me dressed in her clothes and knotted by her dog. It was very embarrassing and humiliating. I apologized for wearing her clothes without permission, told her it wasn't what it looked like, and begged her not to tell anyone. Seeing me in her clothes at first she thought I had wanted to get fucked and had pulled him up on my back and forced him somehow like I had abused her dog in what must have been many times forcing him to do that to me. When I explained what happened and that this was the very first time and she understood it wasn't anything I had planned or wanted to happen, she stayed with me till it was over and made sure that I was okay. I hadn't planned on telling her about my crossdressing and would have never told her or anyone I knew personally what her dog did to me that day. Had she not come home early that day, no one irl would ever know what happened. After that first time getting forced by him and getting pleasure from it, I let him fuck me several times, but we were never caught again. As far as I know, my sister never told anyone else what happened. Not sure how much she knows though. I know she knows about that first time I was fucked by her dog, which was basically a rape, since she caught us ass to ass and knotted, but she might think that was the first and only time it happened. I never told her about the other times with him, or that I liked getting fucked by her dog.