It's James fault! I don't care what anyone says if he hadn't left me alone this whole mess wouldn't have happened! Confused? I should think so. Well I guess I should start at the beginning, my name is Sirius Black and this is what happened:
It was about two weeks before the next moon and the marauders decided to celebrate Halloween together in the shrieking shack with a crate of fire whiskey, needless to say after two hours of drinking the sixteen year old students sense had gone right out the window,and when the dares had started, trouble was just asking to happen. James Potter thoroughly drunk had slurred "Padfoot as a final dare, I dare you to catch your tail!"
As silly dare as that dare was, an equally drunk Sirius stood up taking a wobbly bow accepting the challenge and changed into his animagus dog that looked extremely close to a grim due to the size of the dog. Sirius stumbled sloppily in a circle much to the amusement of the drunk marauders trying to catch his tail before tripping over his own paws and landing in a heap, Remus walked over to the door looking about ready to fall asleep slurred to the others, "Guys enough, come on James, Peter, Sirius we better get back."
They went into the tunnel of the shrieking shack, stumbling and tripping and laughing as they went, Sirius had opted to stay in his dog form and as such kept bumping his nose into things.
They had made it to the tree by the lake when Padfoot had sniffed around lazily marking his territory making his friends laugh harder, James propped himself up on Remus and said "A drunk Padfoot seems to have no human mind whatsoever! I say we leave him here til morning!"
Peter always wanting to agree with James even when drunk let out a "Here here!"
Remus was seeing colourful spots in his vision and just trudged in the direction of Hogwarts half carrying James, Peter trailing behind them leaving Sirius at the roots of the tree. That's where it went wrong. If they had bothered to stay they wouldhave realised they left they're friend with another dog that had been hiding under the space beneath the tree, a female black and white border collie (Sheepdog) that was in her last week of heat.
When James, Remus and Peter had left she wasted no time in introducing herself to the male dog, playfully nipping him, active submissive to him, before disappearing under the tree roots hoping he'd follow.
If Sirius hadn't been drunk he probably wouldn't have acted on the dogs instincts, had he not been left behind he wouldn't have met the she-dog, as it were though, he was and he had.