Elementals are a common feature of modern bestiaries, video games, and RPGs. We have the 16th century alchemist Paracelsus to thank for thinking them up.
The alchemists loved to identify rules that gave the natural world some symmetry. Their intention was to take that sense of order and structure and use it to manipulate the natural world. Alchemy never worked, because they got the order, structure, and symmetry all wrong.
(A side note: when Patrick Blackett, in 1925, recognised that one of Ernest Rutherford’s experiments led to the transmutation of nitrogen into oxygen, they became the first successful alchemists in history.)
Paracelsus was not a successful alchemist. To be honest, his work was foundational for a lot of modern medicine, including toxicology – famously expressed as “the dose makes the poison” – and antiseptics. But he also believed in little invisible spirits that had no immortal souls and dressed in tiny human clothes.
In Liber de Nymphis, Sylphis, Pygmaeis et Salamandris et de Caeteris Spiritibus, Paracelsus wrote about creatures of the earth, fire, air, and water. They were, respectively, the gnomes, the salamanders, the undine (nymphs), and the sylphs. All but the salamanders were Paracelsus’ creation. They could supposedly move through their element without impediment, and died if they left it.
These fictional beings were picked up by a later text, the 17th century Comte de Gabalis. This one was actually a satire, but that didn’t stop people taking it seriously, and the elementals took off. Today they’re an established part of fiction and folklore.
[Thanks to David S. for suggesting this topic.]