Here's what I found: "Melusine is sometimes used as a heraldic figure, typically in German and Scandinavian Coats of arms, where she supports one scaly tail in each arm. She may appear crowned. The Coat of Arms of Warsaw features a siren (identified in Polish as a syrenka) very much like a depiction of Melusine, brandishing a sword and shield."
And: "Melusine is a figure of European legends and folklore, a feminine spirit of fresh waters in sacred springs and rivers. She is usually depicted as a woman who is a serpent or fish (much like a mermaid) from the waist down. She is also sometimes illustrated with wings, two tails or both, and sometimes referred to as a nixie."
Is there another definition of what type of creature this is/was?
A silky, long-haired felt, used for making hats. Usage: 1908 Westm. Gaz. 24 Oct. 19/2 The Parisienne's latest love in millinery is the hat of silky beaver felt that she calls melusine. (from OED online)