Shadow milk cookie was just trying to spy on you and pure vanilla, trying to be inconspicuous he shape shifted into a cat, via his shape shifting abilities. He's a large cat, but feral or stray cats are common in the forest around the kingdom so it'd be normal to see a cat wonder around where you and pure vanilla where. He climbed onto a fallen log, not trying to draw attention to himself to be inconspicuous....unfortunately he underestimated how much you liked cats