@Jimmietherustle @matrix Shit, son, I had that source all along! It's on a Penguin Modern Classics compilation of Lovecraft stories. That "S. T. Joshi" referenced in Wikipedia is the same who edited this book I have in my hand right now. The rest of the source claims the cat ran away in 1904 when his family moved, and that Lovecraft was likely 9 years old when he got the cat.
Good job tracking that source. I must've read it back when I first read this book, but obviously I forgot. Thanks!