Arbor is OK. If he ever learns Latin and finds out that it's feminine, then at least he's learnt Latin.
Arrietty is just plain bad. The whole point of the Borrowers is that they are tiny people who live in our houses and use ('borrow') whatever we discard or mislay for their own purposes. And these are typically not the same as our purposes - why should they be? One of the things they have Borrowed is the name '
Harriet', but as usual they put their own spin on it, making it more theirs than ours. It's been a long time, I don't recall whether there's an explanation of it or not in the books, but it could arise from a misheard
Harriet, or from a child's babyish attempt at
Harriet. Either way, on a full-sized human it's a life sentence which lacks dignity. I love the name
Harriet, and deplore the way this cheapens it: I really cannot see the point.