Undine
Undines are water elementals introduced by Paracelsus in A Book on Nymphs (1566), named from Latin unda ('wave') and classed among the four elemental spirits alongside gnomes, sylphs, and salamanders. Said to lack an immortal soul, they could gain one by marrying a mortal — a motif that inspired Friedrich de la Motte Fouqué's novella Undine (1811) and later the Little Mermaid tradition.