The most prolific resident of Ultra Plant is the ubiquitous Xurkitree. It's difficult to walk even short distances without spotting one rooted in the soil or gleefully skipping along. The landscape itself is often composed of the massive cable-like limbs of the planet's oldest living Xurkitree, curled over hills and winding around mountains. From the sheer size of some wild specimens, it can be safely assumed that when provided with the necessary nutrients, Xurkitree never stops growing and is therefore the largest known organism on Ultra Plant.
Xurkitree's body is composed of a rigid star-shaped head and five distinct tentacles. The head sports numerous sharp points attached to a round core. It is covered with a thick translucent covering reminiscent of plastic or keratin. Inside the head's shell is an organ that produces brilliant light. Wrapped around Xurkitree's tentacles are leaf-shaped tags composed of the same substance as the head and containing smaller photophores. Xurkitree's bioluminescence is bright enough to see from a few kilometers away, and individuals blink their heads and leaves in complex patterns to communicate with one another. X-rays, MRIs, and dissection have revealed optical organs within the shell of their heads.
Xurkitree has three types of limb: arms, legs, and tail. The arm cables split into several smaller wires along the "forearm" and rejoin at the wrist, which ends in a spray of copper fibers that serve as fingers. The fingers are stiff and clumsy for grasping, but excellent for discharging electric shocks in self-defense. Its feet are much more robust. Zygodactyl in structure, the front toes of the feet are fused into a single mass of rubbery skin with two copper claws poking out. The back toes are much more flexible, allowing Xurkitree to grasp uneven surfaces and balance in precarious positions.
Its fifth limb, the tail, resembles the plug of an AC adapter with three sharp copper prongs. The tail's club-like ending is full of tightly bundled wires and power cells that efficiently absorb, transfer, and store electricity for use in metabolism or self-defense. Whenever it needs a quick infusion of electricity, it plugs its tail into the ground to feed on the power stored by the natural energy cells in the ground. When predators come too close to safely attack with energy bolts, Xurkitree will bludgeon them with its limbs. A Xurkitree in good health will favor its tail to attack, using the weight of the plug for extra force. However, the tail is still full of sensitive organs, and a sick or wounded Xurkitree will protect its tail and use its arms or legs instead, especially if the wires in the tail are exposed due to injury.
The metal in Xurkitree's fingers and toes, and by extension running through its whole body, is an exotic biometal composed primarily of copper and a previously unknown metal seemingly native to Ultra Space. The substance is poorly understood. Electron microscope study and particle acceleration have provided astounding results, with the nuclei possessing a five-dimensional structure only confirmed using the space-bending powers of Cosmog. It is currently unknown how this element can even maintain stability, much less interface with the ordinary elements of the periodic table. Leaving Ultra Space and arriving on Earth does not cause these atoms to spontaneously annihilate, allowing Xurkitree to survive comfortably and even happily in non-Ultra Space conditions. The lack of battery acid in the soil is a barrier to their thriving, however.
Conversely, its skin is a powerful insulator used to keep gathered electricity within the body. Xurkitree secretes a latex-rich sap that hardens into a flexible, rubbery skin to protect its metallic innards. Where the composition of the metal veins is alien, the substances in the sap are quite mundane. Certain chemicals in Xurkitree's sap cause the resin to congeal into a pliant coating instead of rigid amber, but they are all composed of familiar chemical elements. Xurkitree's skin is thick but supple, allowing it to bend its tentacles freely while thoroughly protecting its insides from the outside world. When cut, a Xurkitree will bleed. Arteries of sap run through its skin, ready to harden over any wounds and forge new skin in its place. The thickness of a Xurkitree's skin is proportional to its size; the largest specimens have skin estimated to be meters thick.
When Xurkitree uproots itself, it takes on a roughly humanoid stance to walk around. Despite their bizarre and somewhat unnerving appearance, Xurkitree have very warm dispositions on average and will fearlessly approach researchers to study them. They are playful and love to play various games with each other. Some enjoy racing each other across the land, some compete in contests of brightness or electricity output, some wrestle, some throw stones as far as they can manage, and some simply like to dance. Observations in the wild have revealed groups of Xurkitree counting scores and rewarding winners, forming rudimentary sports. Field researchers have befriended wild individuals by offering polished stones as gifts. Several lab specimens were willingly recruited rather than captured and have favorite scientists they need contact with in order to cooperate with tests. Other specimens have specific greeting rituals that must be performed. Some captured individuals will throw violent temper tantrums if their demands are not met, and the labs are not fully equipped to handle the powerful electrical surges these Ultra Beasts can produce.
When Xurkitree reproduce, the shell of the head cracks open to release seeds that drift on the wind. The parent Xurkitree dies in the process. As a result, most Xurkitree do not grow beyond six meters in height because many of them reproduce at similar ages. However, not all Xurkitree choose to reproduce so early, or even at all. Because the species grows continuously, Xurkitree that live and thrive for years beyond their usual lifespan reach gargantuan sizes. The largest individuals observed are the size of skyscrapers, dominating the horizon with tentacles the width of buildings. The land of Ultra Plant can only support a handful of large-scale Xurkitree, and as a result these titanic Pokémon struggle to pollinate each other. Some simply die of old age, while some are lucky enough to become parents, falling over and shattering to release many thousands of seeds. Either way, their bodies become part of the landscape.