On Ravnica, elementals are native to the plane and are generally manifestations of raw mana given shape and substance on the world through various means. In most cases, elementals appear at the behest of the guilds, tied to the ideals and magic most associated with them: both the Boros Legion and Cult of Rakdos have the capacity to summon fire elementals, though their appearance and motifs will differ vastly due to the nature of each guild. Typically, guilds that align more with the natural elements (air, fire, earth and water) are more inclined to have elementals at their behest, but they all have the capacity to create them.
Wild elementals are also a regular facet of life in the city. Eons of urban growth have not managed to quell the appearance of fluctuations of wild magic – it only slowed them down or, in some cases, modified them (sometimes intentionally so). The most common wild elementals that appear are earth elementals, typically in abandoned slums or reclamation zones of the city. Larger earth elementals appear as whole streets or city blocks that have jumbled together and risen as an animate being, an amalgam of fluid stone and rigid architecture. The Gruul Clans are known to coax these elementals to attack city centers for the irony of watching buildings attacking buildings, so to speak.
Perhaps the strangest type of elemental to exist on Ravnica are the weirds, created by the Izzet League. Weirds are characterized by being creatures composed of two different and often-opposing elements, such as water and fire, that still manage to function. These “living paradoxes” are surprisingly stable and often serve the inventors of the Izzet as lab assistants, guards, power sources for Izzet machines and even pets, depending on how and why they are created.
Weirds can be represented in the Pathfinder rules by taking an elemental and replacing one of its special abilities or attacks with the ability of an opposing type of elemental of the same size: for example, taking an ice elemental but giving it the fire elemental’s burn special attack. Playing with the concept on a case-by-case basis represents how each weird is fairly unique, shaped by the whims and talents of its Izzet creator.