Aragog – the cupboard under the stairs – amnesia.
HLotD – So I know that it might not fit in strictly with the HP canon, but just use your imagination. Or assume it's an AU. Your choice!
Harry Potter and the Not-so-Giant Guardian Spider
It was a long time ago. So long ago I find it difficult to remember everything. Time has definitely dimmed certain details which once seemed so clear. But the memories surged back earlier tonight, when I realised exactly who it was in the forest; dark hair, round glasses – and the scar. Those details remained the same.
Harry Potter. So that was his name. Hagrid had not told me. But I see now that this "friend of Hagrid's" was indeed that baby boy I had been told to guard so many years ago. An important task; Hagrid had even gone so far as to perform a shrinking spell on me! He would never have considered such a thing unless the situation were truly dire; in his eyes, the bigger the better. And of course, he was not strictly allowed to do magic.
I still recall the female human's shriek when I emerged from beneath the covers. When the male arrived she feigned amnesia, said it was the sight of the boy which had caused her fall. These people were clearly to be avoided. They spoke scorningly of magic.
Withdrawing to the darkness of the cupboard under the stairs, coming out only for food, I watched the boy turn from baby to toddler. I do not know if he was aware of my presence; I suppose I never will.
He would not remember me anyway, for nearly ten years has passed. Ten years since Hagrid came for me, since I last saw the boy with the lightning scar. The boy who I had once striven so hard to watch over.
Since this realisation that the baby from long ago, and the teenager of today are both one and the same, I have warned my family away from him. I doubt they listen, but I will at least warn Hagrid; warn him not to send the boy into the forest. My family and I are not the only dangers in here….