7
Did Ginny name her pygmy puff after a mistake in the Daily Prophet?
(sh.itjust.works)
There's a good catch, I bet it was!
Also it's a possibility IRL that either Rowling had the name in mind subconciously, or didn't think the Pygmy Puff was important enough to warrant a new name. Or Rowling consciously had Ginny subconsciously name her Arnold, as you say.