The magical elements are said to be peopled by spirits and mythical entities called elementals or nature spirits. These are grouped into four main categories:

Earth — Gnomes, Dwarfs, Elves, Brownies, Hobgoblins, Leprechauns
Water — Undines, Nymphs, Tritons, Mermaids, Mermen, Sirens, Harpies, Salamanders
Fire — Jin, Genies
Air — Sylphs, Faeries, Angels

Elementals are usually only visible to those with clairvoyant sight and are more likely to be seen at night in the mountains or country away from cities -- especially if you are tired or sleepy. Although elementals exist naturally, it is also possible to create one which will exist for a limited time -- no elemental has immortality. A created elemental is called an 'artificial elemental'.

To the Ancients, elementals were the physical explanation of the universe. However some contemporary occultists see them only as symbols for forces and otherwise not 'real' at all. Another word sometimes used for elemental is 'familiar' (usually in medieval witchcraft); the term is ambiguous, as it might merely be an ordinary household pet such as a dog or cat.

SOURCE: The Celtic Connection