Sirenians (also known as sea-cows), as we know them today, represent an order of fully aquatic herbivorous mammals, with a rich fossil record that spans from the Paleogene to the Holocene. In fact, sirenians are the only marine full herbivorous mammals, the others having different diets. Although they are peaceful herbivores, sirenians have fueled an entire legend about their supposed, often imaginary, deeds. Since ancient times, their name has been associated with creatures from the myths and legends of numerous nations, linked to hybrid beings, adopting the appearance of humanoid birds and later, semi-human (especially women), semi-aquatic creatures with scales and a fish tail. Sometimes, they were interpreted as being beneficial for sailors or, on other occasions, malevolent. In this article, we present how mermaids have influenced mythology throughout human history, as well as the scientific aspect of these magnificent creatures. This article does not aim to be a comprehensive approach to the subject, but rather to highlight the most illustrative elements related to the topic at hand. From a scientific perspective, these mammals are challenging subjects regarding their evolution, especially concerning their adaptations to the aquatic environment. From the quadrupedal forms of the Early Eocene, there was a gradual transition to forms that are completely dependent on water and do not return to terrestrial life. Geological history records forms of very large sizes (which became extinct in historical times), as well as representatives of more modest sizes. In Romania, these mammals are recorded from the Eocene and Oligocene. A single report from the Middle Miocene (Badenian) remains highly debatable.