Leshys begin their existence as sentient though bodiless spirits of nature of the sort contacted by spells such as commune with nature. These spirits normally have no way to directly manipulate the physical world, but a skilled spellcaster (typically a druid) can grow a special plant body for one of these spirits, giving the spirit a home to animate.

Once accepted into a body, a leshy's spirit remains within unless the body is destroyed. Leshys do not fear death as many other creatures do, knowing that should they fall, their spirits merely return to the natural world and can be called to inhabit a new leshy body at some point in the future. As a leshy's body dies, the magic animating it unravels in a burst of life energy that infuses its surrounding and quickens the growth of any plants in the vicinity. Some leshys even voluntarily discorporate to save the lives of ailing plants, knowing that their sacrifice may mean the continuation of countless otherwise helpless flora. Regardless of how a leshy dies, leaving the body traumatizes the spirit, and the leshy retains only faint memories of past corporeal existences. Leshy spirits need not return to the same form if bound again to a body. During the course of its existence a single spirit can inhabit any number of different types of leshys. A leshy without a body has no power to affect or contact the material world.

The rites and special materials required to create a leshy's physical form vary between individual leshys. Once the creator assembles the necessary materials, a leshy must typically be grown in an area of natural power, such as a treant's grove, a druidic circle, or a site of pristine natural wonder. A newly born leshy is a free-willed, neutral being, under no obligation to serve its creator.

As a group, leshys share certain common attributes. They resemble plants but with vaguely humanoid appearances, and are always Small. While leshys of the same species share the same general shape, leshys have all the diversity of the plant world in appearance. For example, one fungus leshy may have a classic red, white-dotted mushroom cap, while another might have the spongier and darker contours of a morel, while a third might have the coloration of a pale gray cave mushroom.