Around the trees, where magic flows strongest, grows a variety of small plants, each varied and unique containing special characteristics represented by their physical appearance. Tree Souls, having an inherent connection to nature, immediately understood the basic properties of these plants, which herbs or shrubs give strength, agility, speed, and more. They balance these out when feeding the Elemental Beasts to keep them in their default shape which resembles a blob with small legs and eyes. The Tree Souls have long since discovered through decades of interaction with them that feeding disproportionate amounts to the beasts morphs them into different bigger shapes with stronger abilities that have no inherent use in the forest. After a while of not eating, these beasts default back to their small blob like form.
Back in the old days, certain land spots were cultivated to study the elemental beasts evolution. It was observed that a combination of certain food types paired with elemental husbandry would result in some truly magnificent transformations. YogShru a particularly wise old soul, made the conclusion that if an elemental beast lived in a particular environment while being fed, it would absorb the qualities around him. For example, a beast placed in a rocky cove would develop traits similar to stone; thick hard skin that was hard to penetrate, a larger bulky appearance, and increased strength due to its mass. Whereas a beast that lived in a fiery environment would develop the ability to breathe fire and do tremendous amounts of damage. In his devotion to learning about Elemental Beast evolution, YogShru discovered the 4 main elements that affected their change; Earth, Fire, Water and Wind.
It would take a seismic event though for everyone to see the true potential of these beasts. During times of calamities these evolved creatures form strong psychic links with the Tree Souls allowing them to be controlled in order to help rescue the forest inhabitants. It was from this point on that Tree Souls would often take on a humanoid form to tend and care for these amazing animals, keeping their powers in check and hidden.