On the train ride back to King’s Cross Station, Harry swore that he saw a black dog running along side the train.

At the store with Petunia, there was a black dog sitting outside the car…

…at the zoo entrance…

…in the playground…

…and at the entrance to Diagon Alley.

Harry wondered, repeatedly, as he kept seeing the black dog all over, if he was going crazy.

Finally, as the sweat-drenched days of summer ended, Harry decided enough was enough and approached the first black dog he came across. The black dog, in a vicious and unprovoked attack, bit Harry.