This happened when i was teenager and I've never told anyone since then.
My best mate at the time, owned a Labrador dog. It was always sniffing around me when I called by his house and sometimes it would try and hump my leg.
Staying over one night, my friend and his dad went out to get some fishing gear for the following day, leaving me to look after the house and continue to stay on the game I was playing. I had on only a pair of shorts and t-shirt, as I'd already removed my boxers after showering earlier.
I was in the snug plying my game when the dog came in, growled a little and moved right on up to me. It began to sniff between my legs, then rally began to nuzzle under my arse.
I got scared and moved over, but sort of fell off the chair. The dog pounced on me and tried its hardest to hump me through my shorts. Not getting anywhere it growled again but with more fierceness in its growl. Now shitting myself, I lowered the shorts and instantly it mounted me.
I'd never felt anything up my arsehole before, but that day I got something and then some. Before I knew it, it was inside of me and my arse began to twitch as if it was allowing the dogs cock deeper inside my body.
Now fully on my back, my arse gave some more and the dogs knot slipped into me.
It humped away like crazy after that and was hammering away so fast, it was making me feel weird. It was then I heard my friend shouting out my name and heard his dad saying something about the dog.
The next thing I knew I was knelt up looking into the eyes of my friend and hi dad. The dog didn't care and didn't stop fucking me at all. they stood there with their mouths open watching their dog fucking my arse.
My friend was about to move into the snug, when his dad stopped him and told him the dog would bite me and probably him if he tried to stop it.
Leaving the dog to carry on, I got the feeling back and within a minute I was ejaculating all over the carpet below me.
My friend didn't say anything, but his dad said "Looks like he's a receiver". I thought he was talking about the dog, until my friend later told me he was talking about me.
My friend isn't my best friend anymore, he's much more than that now. He's my lover and it's his cock I now take up my arsehole on a daily basis.
Their dog is still alive, but it's old now and that was only occasion it ever fucked me.
We live together and I service him anyway he wants, and that for him is perfect as he's got such a high sex drive. He mentioned watching someone else fuck me the other day, which is why i'm here. He also mentioned it didn't have to be a human.