Here's what rhymes with roundtail.
pale tail tale jail bail bale dale pail detail male fail mail sale sail trail nail rail veil gale hail shale whale stale vale wail hale braille kale faille wale scale avail entail frail email assail curtail quail snail flail quale bewail derail pigtail topsail grail impale bobtail hightail prevail travail inhale mainsail ponytail unveil countervail monorail taffrail wassail pellmell carrell exhale gunwale foresail martingale nonpareil hangnail lugsail fingernail swallowtail tattletale