I was venturing into the classic mode of Fruit Ninja (usually playing arcade) and swiped an "ultra rare" rather horrible looking fruit for 50pts. Does this ever appear in arcade mode or have I just never spawned it?
No. The ONLY place the ultimate rare fruit appears is in classic mode. It does not appear in arcade nor zen mode. It's really rare but you'll probably get it when you are around 600 points or so.