Giant willow aphids (Tuberolachnus salignus), pictured, are about 6mm long with a dorsal fin.

Source: https://www.dailymail.co.uk/home/article-11769769/Action-plan-Ciar-Byrnes-essential-jobs-garden-week.html?ns_mchannel=rss&ns_campaign=1490&ito=1490

Researchers immediately knew her situation wasn’t good when they saw the unnatural “S” curve in Moon’s spine, from her dorsal fin to her fluke.

Source: https://bigislandnow.com/2023/01/16/what-happened-to-moon-the-injured-humpback-whale-hasn%ca%bbt-been-seen-in-weeks/

You can discern an electric eel from a true eel by its missing dorsal fin, its ability to breathe air and its tendency to lay eggs in freshwater rather than saltwater.

Source: https://www.discovermagazine.com/planet-earth/10-animal-names-that-have-you-fooled