Mine is actually from a Tad Williams novel. In "The Dragonbone Chair," the daydreaming boy protagonist, Simon, spots a small gray cat on a roof where he is wont to go and escape from his chores. The cat startles, and Simon begins shouting, "Scat, you cat! Scatter, you catter!"
Later in the story, the same cat reappears in an oddly touching moment (and this is like eight hundred pages later, for reference) and is referred to as "the little gray scattercat."
I like playing with word sounds, I'm very much a cat person (in the sense that I enjoy keeping cats and in the sense that I'm lazy, curious, easily annoyed by changes in my surroundings, and prone to sleeping all day and waking up at night), and I loved that the cat ended up coming back into the story so long after its completely pointless and irrelevant introduction, so I started using it as my default username most places. (My second choice is "The Feared Avocado," which really is just apropos of nothing. I used to doodle up quotes from "The Book of the Avocado" which promised horrendous fates for those who did not adequately fear the tree-based object of awe and reverence.)
