Jakob Jakobsen, An Etymological Dictionary of the Norn Language in Shetland…
“ Nowadays eder most often means a kind of mythical fish, feared by fishermen, as it is said to be able to perforate their boats when at sea (thus e.g. in the N.I.) . The phrase “to fly like a (n) eder”, to make rapid headway, is used in several places, e.g. on Wests. and in the N.I. From Esh., Nmw., “de eder” is reported as a name for the sea-serpent, and in Un. eder is occas. ”
