Front half is a horse, back half is sea creature (usually a dolphin.) Also Called Hippocampus or the Steeds of Neptune Might actually be The fanciful version above may be based on a misunderstanding of a description of either of the an actual sea-horse (genus: hippocampus) which are only a few inches long, or a hippopotamus. Seen in A common symbolic image among sea-farers, and in heraldry.
In another 14th c. French folktale story of Melusine - half-fairy, half human but due to a curse, once a week she transforms into half woman, half-serpent--a secret she keeps from her mortal husband Raimondin. As Raimondin finds out her secret, she transforms into a full dragon and flies away from him.