Why are there Mermaids on Old Maps?

Internet Sources ---- https://library.princeton.edu/visual_materials/maps/websites/africa/maps-nile/nile.html https://www.fromoldbooks.org/NathanBailey-UniversalEtymologicalDictionary/m/mermaids.html http://thethinkersgarden.com/odd-truths-the-mermaids-of-congo/ RareMaps.com Sources ------ https://www.raremaps.com/gallery/detail/76929