Dan felt around and finally got a grip on the long appendage that kept flopping around through the hole it made in the seat of its jeans. He tensed up at the feeling of grabbing himself in an unfamiliar place, then took a deep breath and started to examine his tail.
It still hurt a bit from the pain of breaking through denim, but that was starting to wear off. Now that he was able to stay still, he could clearly see that what he was holding looked just like the tail on his best friend's yellow lab. It stuck out about a foot behind him and was covered in short, course blonde fur that was a bit fluffy on the underside. He ran his hands over the fur, kind of weirded out by the sensation.
I gotta get this thing out of my pants, he thought, so he let go of his tail and carefully started pulling his jeans down. Gradually felt the tail come back up out of them through the hole. That's a relief, he thought, and his tail wagged halfheartedly a bit left-and-right. That made him curious, and he started moving it on purpose, just to feel the full range of motion. Watching it wave around, it really did remind him of his best friend's dog.
"A dog," he said out loud. "My sister thinks the animal inside me is a dog?"
Dan was insulted but not surprised. Kat was always complaining about how dumb and lazy he was and how he always got out of trouble and how she always had to take responsibility for things that were his fault. She probably really did think of him as more of a housepet than a brother.
"Now I'm actually going to be one," he thought, and his tail drooped a bit.