From Middle English ridder, rydder, from Old English hridder (also as Old English hriddel > English riddle), from Proto-West Germanic *hrīdrā, from Proto-Germanic *hrīdrą, *hrīdrǭ, from Proto-Indo-European *krey-. Cognate with German Reiter.
From Middle English riddren, from Old English hridrian, from Proto-Germanic *hrīdrōną, from the noun. See above.