In its Normal Form, Terapagos is a small reptilian Pokémon that resembles an indigo tortoise. Its head, legs, and tail stem from a cyan, hexagonal shell with the Terastal symbol on its back: a white hexagon with triangular extensions on each corner that connects it to the edges of its back. Its neck and legs each have a cyan four-pointed star and ring rub on them. Its feet each have three toes, and its tail has a cyan four-pointed star at its tip. It has two light blue eyes with rouge rimming and four-pointed star-shaped pupils. It has a diamond-like pattern on its forehead and two diamonds hanging like earrings on both sides of its head. Lastly, it has a "ponytail" made up of three segments of diamond-like shapes. The segments reduce in size as they get farther from the head and gain a purplish tint to them.

Normal Form Terapagos is said to have a splendid aura. It is a small and particularly weak Pokémon, for that reason, it can assume a dormant state whenever its life is put in danger or it needs to hibernate, pulling its head, limbs, and tail into its shell and shrinking in size, which makes itself look like an ordinary jewel. It also has the ability to crystallize the energy in its own body to form around itself a green shield when necessary, even in its dormant state. Through this same ability, Terapagos causes glowing, hard crystals to form around it, even as it slumbers, which gave Area Zero its distinctive appearance. As depicted in Pokémon Horizons: The Series, like other turtles, it is unable to move if it is turned upside down, needing outside help to stand back up. It has shown to have an incredibly long lifespan, with one known to have lived for millions of years while slumbering within its shell.

Terapagos is responsible for the Terastal phenomenon. Terapagos inhabited the Paldea region in a bygone era. It was thought that this species went extinct, with it said to have been caught up in seismic shifts that occurred about 2,000,000 years ago. However, one of the them managed to survive by crystallizing its own body and going into hibernation in the Area Zero Underdepths, where it laid low until the present day. According to Briar, Terapagos is made of Terastal energy. Heath is the one who discovered Terapagos and coined its name. With the power of its crystals, Terapagos can bombard its foes with countless star-shaped projectiles.

Terapagos also has the ability to alter timelines in some capacity, as seen in the core series games and animated series. It is capable of bringing humans, certain Pokémon, and objects forward from other points in time or from alernate timelines altogether. While using this ability, a mist appears around the affected area; once it disappears, whatever was brought is sent back. As revealed in the Pokémon Horizons episode The Plan to Capture Rayquaza, Terapagos has the ability to call out to other Pokémon with a specific energy signal. As seen in several Pokémon Horizons episodes, Terapagos also has the ability to create visions of the past through the use of its own memories. Due to the Terastal energy its body produces, Terapagos was responsible for allowing advanced technologies, such as Tera Orbs, the professor's AI duplicate, and the "time machine" to exist in spite of human limitations. Tera Orbs directly exposed to the energy from Terapagos will gain the ability to Terastallize Pokémon without the need to recharge. Terapagos also has a strong connection to the Stellar type.

Forms

Terapagos has three forms: Normal Form, Terastal Form, and Stellar Form.

Terastal Form Terapagos

Terapagos can absorb the energy around itself in order to crystallize it and clad its body in a larger, sturdier shell. It also causes the rings on its legs to transform into fur, which forms a mane and three tails, thus transforming into its Terastal Form.

Terastal Form Terapagos is a reptilian Pokémon that resembles an indigo sea turtle. It now has two patches of voluminous, shiny fur: one near its head, vaguely forming a mane, and the other where its star-tipped tail used to be, vaguely forming three separate tails. The color of its fur varies between a greenish yellow and sky blue. In dark places like caves, this fur glows bright blueish-white. Its hind legs are fully covered by the fur, while only the toes of its front legs are visible, barely poking out of the mane. The eyes remain unchanged from the Normal Form's ones, but they are seen gleaming with the same hue as the fur. The diamond-shaped earrings and ponytail seem to have merged with Terapagos's head.

Terapagos's shell is composed of eighteen irregular pentagons on its surface, each decorated with symbols of varying shapes and colors in a way to resemble the eighteen type icons. The symbols morph back and forth to abstract shapes. On the center of its shell is a cyan, hexagonal gem adorned with the Terastal symbol, much like the small crystal shell of Normal Form Terapagos. The sides of its shell also have three obtuse pentagons each on the rim of the shell that are a solid light blue color. Due to its lack of legs, Terapagos needs to float to move around in this form.

Normal Form Terapagos enters its Terastal Form when it is either in danger or about to battle. Its shell is composed of crystallized Terastal energy and holds the power of every type, allowing it to distort type matchups and render all damage-dealing moves as ineffective. The shell then absorbs the move's energy and transfers it back to Terapagos. When using specific beam-like attacks, Terapagos withdraws its hands and head into its shell, shapes its mane in a way it resembles its three tails, and flies up, shooting the attack from the cyan, hexagonal gem in the center of its shell. In this form, it is known as "the Indigo Disk", due to its indigo coloration and disk-shaped shell.

Stellar Form Terapagos

When exposed to a Tera Orb, Terapagos will Terastallize and become Stellar-type, thus transforming into its Stellar Form.

Stellar Form Terapagos glows and surges with power and is fully coated by a rainbow-colored aura. All the icons on its shell become cyan, but the rest of its true body is identical to Terastal Form Terapagos and is seen floating above a large, crystal, indigo dome covered in layers of overlapping hexagons. Under the dome, there are four crystals positioned like the legs of a turtle. The gem on the center of Terastal Form's shell has expanded outward into the shape of a crown bejeweled with rhombuses of varying colors. Atop of the crown-like structure lays a miniature version of Normal Form Terapagos and, above it, the Terastal symbol. Eighteen hexagonal gems orbit around Stellar Form Terapagos's dome, each bearing the icons and colors of one of the eighteen types. Terapagos needs to float to move around in this form as well.

This form is said to resemble the world as the ancients saw it, supposedly due to the dome beneath it. Meanwhile, in the Scarlet Book/Violet Book, Heath describes it as an entity, unsure if it is a Pokémon, or even alive to begin with, however, he claims that viewed as a whole, it resembles a mysterious, brilliant disk, and even looks like a planet floating in space. Its Terastal energy becomes so abnormally amplified that it actually appears to lose control and lash out due to its energy output. Due to having so much power, Stellar Form Terapagos may cause cracks around it during its transformation and even risks wreaking havoc to Paldea's ecosystem. While in this form, it was shown to resist going into a Master Ball, destroying it in the process.

Stellar Form Terapagos is capable of absorbing Terastal energy from other Terastallized Pokémon, causing their Terastallized state to be undone earlier than intended. With it, it can create multiple Tera Shields and even change its type amidst battle, though this seems to be limited to only three times per battle. The hidden powers within Stellar Form Terapagos are capable of reducing to zero all effects of weather and terrain. It has shown to be capable of shooting huge beams of pure Terastal energy at its foes. When using specific beam-like attacks, Stellar Form Terapagos will use all of its hexagonal gems in a certain formation to shoot the beam from it. As seen in HZ075, Terapagos can transform into its Stellar Form without the need of a Tera Orb. In the same episode, Terapagos was seen sharing its power with other Pokémon, which allowed it to seal an entire area within a giant, protective shield. This however, will drain away its energy and so Terapagos will change into its dormant form, hybernating for 100 years. In this form, it is also known as "a Disk Pokémon", due to its disk-shaped shell, and "the Hidden Treasure of Area Zero", due to its status as the secret treasure that was sought by the Paldean empire.

(Source: Bulbapedia)