I'm going to start off by saying that Pixelprobablylikescats.
Something that I've wondered about for a while now is that, Cave Story, despite being a huge game that Pixel worked on for years, is nearly cat-free. Off the top of my head, I can only think of Monster X (who only appears on the screen for maybe half a second as a cat) and that little Debug Kitty sprite that's hardly used. He developed a whole race of sentient creatures that don't really look like cats (They look a bit more like them in the Wiiware version, but that's not the point).
I can only wonder as to why he made the decision not to include them, as I'm pretty sure he didn't just forget.