Glastrier is an equine Pokémon with a white coat and has light-blue ice covering parts of its body. Its mane consists of ice spikes, with three rows behind its neck and a single large spike between its ears. Both have ice behind them, connecting to the mask of ice that covers the left side of its face, which is noted to be 100 times harder than diamond. Its eyes are white with no visible pupils. Its thin tail is covered in ice that has four spikes jutting out from the sides. Its legs have ice covering its hooves and fetlocks, with the ice on its forelegs extending to the knees.
Glastrier possesses incredible physical strength and extraordinary destructive power. It is noted to be an aggressive Pokémon that forcefully takes anything it wants. Glastrier generates extremely cold air within its body and emits it from its nose and hooves. While Glastrier cannot move fast, it can freeze the ground to allow it to slide across the ice in a flash. Glastrier's legs allows it to travel through the snowiest of lands and worst of roads. It enjoy challenging any strong opponents it finds worthy of battling.
Glastrier was known to be very proud and arrogant, preferring to serve those it deems the strongest. It once ruled over the Crown Tundra like a lord, terrorizing its residents for intruding on its territory. This lasted until it was defeated and tamed by Calyrex using the Reins of Unity. It has a fondness for carrots, particularly Iceroot Carrots.
Glastrier together with Spectrier may also be based on the Horsehead Nebula, a horse-shaped nebula near the constellation Orion. Glastrier itself may also be based on the Monoceros (unicorn) constellation, which also borders Orion. This would correspond to Calyrex possibly representing Orion itself or Lepus, the "hare" constellation located right below Orion, as well as Zacian and Zamazenta possibly representing Canis Major, which borders both Monoceros and Lepus.
Both Glastrier and its counterpart Spectrier's relationship with Calyrex appears to be based on the legend of Brian Boru and the Pooka. Legend has it that Brian Boru, the High King of Ireland, is the only person to have ever ridden a Pooka, a mischievous faery in the form of a black horse. Brian tamed the Pooka by using a special bridle that contained three hairs from the Pooka's tail and successfully staying on its back through a wild ride. The king then forced the Pooka not to torment Christian people and to never attack an Irishman ever again. This is similar to how Calyrex, a Pokémon known as a king, can only ride its loyal steed, Glastrier or Spectrier, using the Reins of Unity woven in part from hairs from the steed's mane and had to prove itself by riding through the steed's bucking. It also relates to how, in taming this steed, Calyrex stopped the steed Pokémon from tormenting the people of the Crown Tundra.
The appearance of Glastrier may be based on the unicorn, the national animal of Scotland – which is the country that the Crown Tundra is based on. The unicorn in mythology is also said to be extremely wild and can only be tamed by the pure of heart; Glastrier is noted to be very aggressive and only the graceful Calyrex was able to tame it. Glastrier may also be based on Shire horses and Clydesdale horses, draft horse breeds from Britain. Its large ice-covered hooves may be a stylization of the horse breeds' large feathered hooves. In addition, Glastrier's tail shape may be a reference to an equestrian tail guard worn over the dock of a military or sport horse, leaving the tail fan spreading.
Glastrier may draw inspiration from legendary horses in Celtic mythology, such as Hengroen, a stallion owned by King Arthur in the Welsh tale Culhwch and Olwen, or Liath Macha, a grey-colored chariot horses of the hero Cú Chulainn in the Ulster Cycle of Irish Mythology. It may also take inspiration from other water horses in Celtic and Nordic folklore, such as the nykur, the water horse of Iceland and the Faroese Islands. This water horse is said to be white in color, and according to Icelanders, the sound of ice cracking on frozen lakes is the sound of the nykur's neighing, which may also connect to Glastrier's Ability, Chilling Neigh. It also bears similarity to Glashtyn, a water horse or goblin of Manx folklore whose name derives from glas(s), the Celtic word for 'stream'. The Glashtyn is sometimes called the "Howler", because its eerie cry is said to warn of approaching storms, which may also be referenced by Glastrier's Ability. The Glashtyn is also said to fear fire, which Glastrier is weak to due to its Ice typing.
Alternatively, Glastrier may draw inspiration from the mythological Four Horsemen of the Apocalypse, Glastrier representing the white horse of Conquest given its belligerent nature and purported desire to take anything it wants by force. It may also be based on the white-colored knight chess piece. Together with Spectrier, it may also be partly inspired by Ema plaques and horse effigies in Shinto shrines that pray for good harvest.
Glastrier may be a combination of glacier and destrier, a type of medieval war horse.
Blizzapos may be a combination of blizzard and ἵππος híppos (Greek for horse) or horse.