An Elder God is, in simplest terms, a god of sufficiently higher power to its contemporaries that it can be considered in a category of its own. An Elder God is, or should be, to gods what gods are to mortals- an entire tier of unimaginable power higher than the rest. Though no gods have ever been formally identified as elder gods, the concept of Elder God Theory is one that has persisted throughout the centuries, though it has often been seen as an exercise in futility, the mentally deficient cousin of philosophy, science and magic.
The definition of an Elder God changes from scholar to scholar. To some, an Elder God is the patriarch of a given pantheon. If this is the case, then the known Elder Gods would be Tyrrus, Odin Allfather and The Lightbringer, as the heads of the pantheons of their respective continents. Janus could also be included in this list, if the Church of Engelain interpretation of the Tyrrus/Janus relationship is true. Support for this particular strand of Elder God theory comes from one of the most ancient Kal'sean myths, in that the heads of every pantheon on the planet combined in order to seal away The Demon Lord, who in this interpretation would be the Eldest of the elder gods- the First God, if you will.
Others within the Elder God theory community dispute this train of thought, as it does not include beings such as The Great Stallion and Sarcoseth, who are singular deities worshipped by the centaurs and avians, respectively, yet do not feature on the Elder God list. However, if they were to be included, then the number of potential Elder Gods goes up astronomically- after all, who is to say that Fiara, Ciera and the other planetary goddesses are not the heads of their own pantheons on their own planets? If this is the case, then this definition of Elder is arbitrary, for the power difference between Ciera and Tyrrus is astronomical.
The next idea of Elder God is that an Elder God is the physical personification of the realm it is birthed within. In this line of thinking, Janus was not created by Tyrrus, but was instead born to the Halls of Janus. If this is the case, then the list of Elder Gods is reduced drastically, to Tyrrus of the Mortal Realm, Janus of the Halls of Time, The Demon Lord of the Demonic Halls and an unknown entity for the Underworld. This can further be extended to the idea that only a being powerful enough to create its own realm can be an Elder God, and so Tyrrus therefore is Elder not through birthright but through merit, by creating Heaven, and Soahc's creation of the Demonic Halls would change 'The Demon Lord's use as a placeholder, as many are sceptical of such a creature's existence.
The simplest explanation set forward is that an Elder God, similar to the Elder Demons of the Demonic Realm, have existence for as long as there has been existence. The Elder Gods are those that are not created, those that simply be. This handily includes gods such as the Great Stallion and Sarcoseth whilst excluding Fiara, Ciera, Soahc and others. All in all though, the mere existence of elder gods is at best speculation, at worst the nonsense ramblings of those with little else to do with their spare time. After all, if Soahc could match Tyrrus in warfare, then what does it matter which god is 'elder' and which is simply god? If a demigod is as powerful as a god, then a god can be as powerful as an elder, and therefore the distinction is meaningless, a matter of semantics, a war of technicalities that need not be waged.