Disclaimer: I don't own these characters. They are Mrs. Rowling's creations. Likewise I am not Greek or roman so I take no claims at their myths. I solemnly swear that I make no money doing this and so you shouldn't sue me.
Authors note: I try to keep everyone as in character as humanly possible and I try to stick to both Rowling's description of the characters and the mythic descriptions. This can be hard sometimes but cut me some slack and don't flame me. This is gonna be my pride and joy when it's finished.
The legend of Sirius and Remus
Part one: A falling star
It is known of Orion a hunter who was laid into the weaving of the constellations with many a mythic hero. He challenges the creatures of the stars with one by his side in blind faith. His main antagonist is The Scorpion called Scorpio and their battle has always put Orion's life at danger until the creature of blind faith tips the balance and Scorpio is chased once again from the sky. It is like this every year because Apollo's anger at Orion is forgotten.
The anger would have been forever buried should it not have been the fault of the mortals below who have abandoned their temples to the old gods and moved into a dark time fighting against barbaric nations. The mortals the gods love so have fallen in battle and the new race does not honor any god like Apollo and before he dies from lacking he wishes to show his powers. He does so in the ways a god of the sky is trained in miracle working. There are eclipses and meteor showers under his power but none to woo some followers from the stubborn rouge bands. Apollo's strength is failing as the rise of Scorpio is approaching. In one last burst of power and anger and revenge Apollo knocks the scale tipping holder of blind faith from the sky and into the dark forests that hold creatures of the river Styx.
The Dog Star has fallen. The inky blackness covers his guiding light and there is no Lucifer's torch to relight it. There are miles and miles from the heavens to the forests and during this fall a god takes pity on the falling Sirius and bears to him the body of a mortal and a good wishing. There falling forever but not long enough a dark body falls to the forest floor cushioned by it's own immortality. Here lying prone and unconscious the goddess Selene takes pity on Sirius and grants him back his canine body for when he wishes it and when he needs to heal. Her full face looks upon the other creatures she has helped and picking the one she feels most effective send him in his bondaged form to aid the fallen Dog Star.
In the stars though the battle rages as Apollo's spirit falters and succumbs to nothingness. Orion is loosing and loosing until even in starry immortality without Sirius he is lost. Scorpio takes his place but the barbarians give a new legend of one without a warrior in the sky but to one of a dragon or some beast and as far as their memory tells it has always been that way and there never was a brightly shining dog star.
Selene's servant has reached the fallen star by the time Orion is dead and it is soon evident that he is the full reason this forest is so feared. It is a tawny wolf with amber eyes and a white garment in its mouth that stands before the fallen star. Though no mortal would see the large black dog that lays at the wolf's feet this wolf can see it clearly. It normally would have spent the night running through it's dark territory seeking vengeance upon the mortals this one day a month but tonight it aids a creature not unlike itself. It can sense a human part, a god part, and a canine part to this creature just like itself and with a soft laugh from Selene the wolf is stricken with love for this kinned creature. It lies down beside the fallen star and cares for its wounds as best it can, giving it the best of canine affections. As the moon Selene sunk below the horizon once more both creatures were asleep tucked against one another shifting slowly into their human shapes.