Apologies if I seem ornithologically obsessed at present -- however: Wiki discourses at considerable length, about the possible etymology of origin of Arnold's name -- the "old" part meaning "nook / corner"; and the "arn" element, perhaps from "herne" -- old name for heron; or perhaps pertaining to -- present many centuries ago -- the white-tailed sea (and "river") eagle, otherwise known as the "erne". Re one of these bird species: Herongate (south Essex), near Brentwood, is reckoned perhaps -- though most likely not -- to take its name from the long-beaked-and-legged grey fish-loving creature.
(Further and incidentally on this theme -- Martins Heron, Berkshire: has for certain, nothing to with martins or herons !)