Once upon a time, Siren no Orakio created a an exotic lifeform, and the similarity to the concept of elementals was striking. And so was initiated a discussion titled "Elemental as Life Forms", most of which you will find in the scrolls below. (Note that newer submissions may have overshadowed these ideas by far.)
This Codex serves a larger purpose - it collects the alternative looks on creatures of the elemental kind. For games of a more scientific bent, it can prove useful to have such a lifeform at hand, strange but believable in its context, without referring to anything magical, other planes of existence, and so on. The supernatural realms will surely have similar creatures, be they made of magic itself or just fantastically ‘natural’. For more fun, you could even pretend, that one explanation is valid, while another is true in fact!
Enjoy and add your thoughts.
Additional Ideas (3)
The elementals of Air could live in the higher layers of the atmosphere. Their extremely thin body looks very akin to a jelly-fish. They are transparent and almost invisible to our eye, but humanoids usually don't get to see them anyway. Another lifeform based on magnetism, it should be noted it is slightly radioactive.
Once down on the ground and magically protected, they can fly freely, but are compressed by the cruel pressure together, thus made visible and dangerous in their own way.
The Water elementals exist, as you have already guessed, deep in the Sea. Again is their body composed out of specially charged molecules of water (or air, or metals respectively), with magnetism being the "soul" of the beast. Conversely to the Airy being, the magic holds it together, as it would simply flow apart and be unable to control itself in our low-pressure environment.
For an Earthier sort, I would say this: Deep within the earth, in the places where geodes grow, some of these crystalline masses have tripped over the next step into complexity. Living on a time-scale completely incomprehensible to human beings, these creatures gain energy from the sonic vibrations within the earth's crust, and they manipulate their environment over a geologic timescale via micro-manipulation of gravity, and the controlled emission of radioactive heat. For one of these creatures to move feet takes literally centuries, if not millenia.
To summon an Earth Elemental is perhaps the very most difficult to practice, for not only must the conjurer pull the creature across space, but he must also alter the flow of time such as the creature perceives it, or it will never be able to react on a human scale.
For the Fire Elemental: These creatures are transient expressions of the exquisitely complex thermal patterns of the flame. These ephemeral constructs cannot exist on their own for a sufficient time to develop real life, at least without aid. To exist as a recognizable entity, the conjurer must prepare a 'blank slate' of magical essence, and he must copy a split seconds grasp of the thermal pattern into the essence. The most curious and grateful of the elementals, these creatures have no previous life-style to be attached to, and are the most likely to be fiercely devoted to their masters.
To conjure a fire elemental requires an exceptionally complex thermal pattern, which almost exclusively from great bonfires, though summonings from spectacularly ill humanoids have been reported.
Several of the creatures presented here may not only listen to a summons, they may not even survive the summoning, due to dramatically different conditions where the summoner is located. A Summon Elemental spell needs to do the following things, that tend to become progressively harder:
1. Call an Elemental.
2. Coerce the alien mind to cooperation.
3. If necessary, bind protective magics around the creature, making it last long enough to serve - or be able to perceive the world in the first place.
The death of an Elemental as we see it, is the moment, when the supporting magic runs out or is disrupted by outside forces. The elemental is likely to desperately seek its home conditions, and return. If it is unable to, it may in its death throes attack its "caller", or anyone nearby.
The very first Elementals must have been discovered through chance, perhaps by those with great powers of mind, that contacted these beings living deep down. After creating the necessary magical protections, the Elementals could discover our world. But even if not very smart, a bond could form, and the first Elementalist could call his friend/pet for help. This knowledge has spread... and the spells were gradually improved... until the day came when practically any powerful wizard is able to call upon this help, enslaving the being in the process. No wonder that Elementals seem so hostile to this world - some like to explore it, but being called on every whim of cranky old men can annoy every type of creature.