It's also worth noting that in the context of Crown Tundra's campaign, Calyrex couldn't even remember what its steed originally looked like. It lost so much of its power over the years and even its memories of what it did with its steed are unclear even to itself.
For the sake of the game's campaign you are only allowed to choose one of the steeds but it is entirely possible in reality that Glastrier was indeed a "draft horse" during its time and that Calyrex had both horses and they each served different purposes for it. Calyrex lost most of its power and with it most of its memories, even to a point where it is easily impressed by modern humans and their capacity for innovation because its loss of power left it estranged from the Pokemon world for a looooong time. So of course, it can easily be perceived that Glastrier did act as a draft horse once upon a time while Spectrier was a fast war horse back then.
Also more importantly, stats aren't everything, but looking at the SwSh website and Glastrier's page, it also says this:
Nimble movements are not Glastrier’s strong suit. But it can use the cold exuded from its hooves to freeze the ground, letting it shoot off sliding across the ice to close distances in a flash during battle.
So Glastrier hypothetically *can* move quickly if it wants to and act as a war horse as well. It can freeze the ground into solid ice at will and move more quickly across distances by creating an ice field then glide across it. So if Calyrex did use Glastrier as a war horse, it can create ice fields with Glastrier and use the slipperiness of ice to glide across the battlefield at a quick pace.
As for the stats, they definitely seemed to want to do an "antithesis" stat lineup between Glastrier and Spectrier: Glastrier is slow, bulky, and a physical attacker, while Spectrier is fast, frail, and a special attacker.
I'd also like to note on the competitive side of things that Glastrier took off very well in VGC and Doubles, and Game Freak definitely looks at the Doubles format as a priority for many mons they design for competitive battles. In Doubles, Glastrier makes a fantastic Trick Room sweeper, as its slow Speed allows it to outpace just about everything under Trick Room, it has the bulk to take a hit, and it can immediately give itself a +2 Attack boost with Weakness Policy using its bulk and the weaknesses of its typing to take a super effective hit and trigger it, all the while it has Chilling Neigh to boost its Attack each time it gets a KO, making it a deadly Trick Room sweeper. It also has a very good offensive movepool with the coverage to back up its excellent Ice STAB. It is much less effective in Singles because Trick Room is a less effective strategy there, but in Doubles Glastrier can excel as a Trick Room sweeper, especially since it can always work with a partner like Cresselia or Dusclops to help it set up TR. It has its place, and is very much usable in its own right, but its capabilities are by nature more suited to Doubles than Singles.
