Brother, dog and I
Little backstory: my parents divorced when my brother, Bill, and I were four and six. We lived with mom, and she worked a lot, so Bill and I spent most of our time just the two of us.
When I was ten, I walked in on twelve-year-old Bill getting his cock licked by the family dog, Molly, who was a Great Dane. I'll say this to his credit, he never, in a hundred years, would've abused her or forced her to do this. Neither of us ever would have; we loved her like a sister. And to her credit, she was very skilled at what she was doing. I had to say his name twice to get them to realize I was there, and by then, I didn't want to get him in trouble; I wanted IN!
From then on, there was no stopping us; Bill and I tried everything we could think of, both alone and with Molly. She loved licking our cocks, and any time we could pry her off us, it was just to suck each other off. She didn't seem interested in having her vagina played with or penetrated, unless she was in heat. We learned quickly that when she WAS in heat, she'd pretty much jump at us pussy-first, so for a week out of every month, Bill and I fucked her until neither of us could walk, and we still needed to raid our mom's sex toy stash for a dildo because Molly was insatiable. We'd spend almost every unsupervised hour of that week in a pile, usually with one of us sixty-nineing the dog while the other one fucked her.
This went on for six wonderfully exhausting years until Molly died at a ripe old age, the only occasional drawback being Bill every once in a while convincing himself he raped me, or molested me, or whatever. I tried to convince him that I loved it as much as he did, but he was, and now is(and we're in our twenties), still sometimes convinced he raped me.