While it is not known to evolve into or from any other Pokémon, Calyrex has two additional forms, which are activated by using the Reins of Unity on it and either Glastrier or Spectrier, resulting in a process that removes Glastrier or Spectrier from the party until the fusion is undone. It will then become Ice Rider Calyrex (Psychic/Ice with Glastrier) or Shadow Rider Calyrex (Psychic/Ghost with Spectrier).
Calyrex is a bipedal Pokémon with legs approximately twice as long as its torso. Four prongs grow from its head to support a green, transparent bud resembling a type of crown, much larger than its head. Inside the crown-shaped head is a white pistil at the center. A ring of green spheres, marked with an inlaid series of blue and white triangles, adorns its neck. The crown-like bud on Calyrex's head glows bright green when it is using its powers.
Calyrex is said to have ruled Galar in ancient times as a merciful king. Despite its delicate and slight appearance, it can move with grace and dignity. It is known to be highly intelligent; Calyrex is one of the few Pokémon able to fluently understand and telepathically communicate the human language. It also has the power to see the past, present, and future of any kind when at full power. One legend said Calyrex used its foresight ability to save creatures of a forest from a meteor one time. One fairy tale even mentions Calyrex stealing the body of a human that causes mischief. Calyrex has the ability to take control of a human's body in order to communicate and speak telepathically when at full power.
It has been referred to as the "King of Bountiful Harvests" due to its role in bringing lush vegetation and harvests yearly. Calyrex's right hand can cause the land to be covered in verdant grass and blooms, while its left hand allows it to make fields of fruit ripen and become heavy. Calyrex can create either Iceroot or Shaderoot Carrots, depending on the field where the Carrot Seeds are planted, with a special dance. Calyrex can also create the Radiant Petal needed for the Reins of Unity and with enough energy, even a complete flower. It can provide healings or blessings such as mending the heart. While it shows no mercy to those that stand in its way, it will heal the wounds of a foe afterwards. Calyrex is powerful enough to move an entire forest and its Pokémon to a new location.
As mentioned in the Pokémon Adventures Manga, Calyrex participated the first Darkest Day where it met Eternatus for the first time.
Calyrex has two alternate forms: Ice Rider Calyrex and Shadow Rider Calyrex, which are formed by uniting with Glastrier and Spectrier, respectively.
When it is riding both Glastrier or Spectrier using the Reins of Unity, it regains its full power and original form, causing the green fur on Calyrex's arms and back to extend, resembling a white and green cape. Calyrex's crown-like bud on its head glows bright green constantly while in this form.
Calyrex has the power to tame and combine with Glastrier or Spectrier when using the Reins of Unity. This allows Calyrex to be at its full power and potential once again like it was before losing the faith of the humans. Calyrex also can heighten Glastrier's or Spectrier's power with its psychic energy. This in turn allowed Calyrex's psychic powers to manifest up to a 30-mile (50 km) radius. When riding either Glastrier or Spectrier, it ran all over Galar to bring green within the region's wastelands. Calyrex gets its powers from the faith of the people who worship it, and slowly loses its power if people start forgetting it or if Glastrier or Spectrier abandon it.
Ice Rider Calyrex
By combining with Glastrier, it can can emit a cold temperature up to −508 °F (−300 °C). Glastrier's endurance also increases to the point it can run up to seven days and nights. This allows Ice Rider Calyrex to smash and freeze anything in its path no matter the difficulty of the road. Ice Rider Calyrex can use its psychic power to transform Glastrier's intense cold into gigantic icicles. It can use the icicles as other arrows to fire or lances for charging. The icicles are powerful enough to blow holes in rocky mountains and allows the duo pass through it completely.
Shadow Rider Calyrex
When fused with Spectrier, it becomes swift to the point it can run a thousand miles within a single day. The strength of Spectrier's spectral power also increases. Shadow Rider Calyrex can infuse Spectrier with its psychic powers in order to create countless small will-o'-the-wisps. It uses the wisps to rain down on others like meteors before it steals the opponent's life-force with burning heat. It can also transform the steed's spectral aura into a black mass. This mass can absorb the life-force and souls of any being's energy like a black hole.
Calyrex debuted as a silhouette in PASS37 and saved Bede from freezing, but later, it properly appeared in PASS38, taking control of Peony's body so it can communicate to Peonia, Bede and Henry. It recognizes both the Rusted Shield and Eternatus's poison, revealing the two Legendaries share some history. It then offers to help the trio find the Rusted Sword if they help it recover both its memories and powers, after the three agreed to it, they left to ask around the village for who might still remember the King of Bountiful Harvests and this results in it regaining some of its memories, remembering it once has a loyal steed. They manage to gain information of said steed and Calyrex remembers they need to attract it with carrots, which would grow into either Iceroot or Shaderoot Carrots depending on where it'd be planted, the children chose the Old Cemetery's crop and Calyrex uses its powers to make it a Shaderoot Carrot grow rapidly, luring its steed, revealing itself to be Spectrier.
In PASS39, the four manage to hide with the carrot, causing Spectrier to give up and gallop away in the direction of Freezington, stepping up for its people, Calyrex reveals itself to them and due to them believing in it once again, Calyrex gains enough power to blast Spectrier away, forcing it to flee. Later, Calyrex talks to Bede, Peonia and Henry about the Reins of Unity, it remembered that Freezington's people used to craft these as a sign of their bond, but as time passed, this tradition was forgotten, using all of its remaining energy to create a Radiant Pedal, causing it to faint, using the Radiant Pedal along with a tuft of Spectrier's black mane, it enabled Henry to craft new Reins of Unity for Calyrex. In PASS40, the group travels to the Crown Shrine where Spectrier resides, they hide away and attract Spectrier there with the Shaderoot Carrot, but when Calyrex attempts to use the Reins of Unity on it, A GigantamaxCorviknight and Butterfree appears above the shrine due to the Darkest Day starting once again. Determined by its allies willingness to help it, Calyrex takes the reins and quickly mounts Spectrier, conquering it and both unite into Shadow Rider Calyrex, who instantly gallops away to protect the people of Freezington, easily defeating the three Dynamaxed Pokémon there and recovering the Rusted Sword for its friends as promised, fainting once back to them due to the poison still covering the sword.
Calyrex last appears in PASS41, saying its goodbyes to Bede and Henry as they finally leave the Crown Tundra with the sword and shield to save Galar once and for all.
Calyrex has two alternate forms, Ice Rider Calyrex and Shadow Rider Calyrex, which were introduced in The Crown Tundra expansion for Pokémon Sword and Shield. The Reins of Unity can be used to combine Calyrex with either Glastrier or Spectrier, with the former resulting in its Ice Rider form and the latter its Shadow Rider form; the same item can also be used to undo the fusion and revert Calyrex to its standard form. Each fused form has its own Ability that combines Calyrex's base Unnerve Ability with either Glastrier's Chilling Neigh or Spectrier's Grim Neigh, but in both cases, the combined Ability is called As One.
Ice Rider Calyrex is the result of combining with Glastrier and is Psychic/Ice. After combining, Calyrex will learn Glacial Lance. Its cry is a combination of regular Calyrex and Glastrier's cries.
Shadow Rider Calyrex is the result of combining with Spectrier and is Psychic/Ghost. After combining, Calyrex will learn Astral Barrage. Its cry is a combination of regular Calyrex and Spectrier's cries.
The forms differ in base stats and learn different moves from each other. If Calyrex separates from Glastrier or Spectrier, it will forget Glacial Lance or Astral Barrage, along with any other moves it knows that Calyrex can't learn in its base form. If this process would leave Calyrex without any moves, it will learn Confusion.
If Calyrex is combined with Glastrier or Spectrier, it cannot be traded, released, or deposited into Pokémon HOME. Due to this and the mechanics of the Reins of Unity, it is only possible to have a single mounted Calyrex at a time.
Ice Rider Calyrex is the heaviest Ice-type Pokémon.
Ice Rider Calyrex achieves the lowest temperature of all Pokémon, −508 °F (−300 °C), which would be a negative temperature that is impossible in the real world, since it is below the absolute zero temperature, −459.67 °F (−273.15 °C).
Unlike other Pokémon, when Calyrex Dynamaxes, it gives off a blue aura instead of the usual crimson one. Calyrex is the only Pokémon to do this.
Calyrex is the only Pokémon capable of fusion with another Pokémon that can be captured in its fused state. Previous Pokémon, such as Kyurem and Necrozma, could only be captured when separated into their individual components.
Calyrex is the only Pokémon that can change between alternate forms at will to have the list of moves it can learn from TMs, TRs, and tutor moves change between them.
When Calyrex unfuses, it will forget not only its signature move, but all moves that are unavailable to its base form.
Calyrex's base typing makes it weak to the types of both its mounts.
Calyrex's head petals are actually transparent, and an antenna/a pistil like feature can be observed if looked from above. The bulbous Crown of Calyrex ,the grass typing and the supposed pistil might reference its attribution as the king of bountiful harvests.
Calyrex may be based on an arctic hare and a crown. The large bulbs on its head and around its neck resemble berries or flower buds. It also bears some resemblance to fictional taxidermy creations, such as the American jackalope or the German wolpertinger. Its beads around its neck each bear the Hōjō clan symbol. Conceptually, it may be based on pre-Victorian depictions of fairies and elves, or any one of a number of horned deities from various European pagan religions, such as Cernunnos or Herne the Hunter.
It may also reference the idea of the king in checkers or chess. Furthermore, its silhouette is reminiscent of a thistle, the floral emblem of Scotland, which is the real-world location the Crown Tundra is based on. Its fusion forms appear to be a reference to cavalry, or the folklore of the Wild Hunt. Calyrex might possibly represent Orion or Lepus, the "hare" constellation located right below Orion, what corresponds to Glastrier and Spectrier which could be based on the Horsehead Nebula near Orion.
Calyrex may be a combination of calyx (Latin for bud) and rex (Latin for king) or carex. It may also incorporate cavalry, referring to its ability to mount Glastrier or Spectrier.
Budrex may be a combination of bud and rex (Latin for king).