Es gibt/gab Pferde.
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There are definitely horses in Tyria, and the NPCs mention them both in GW1 and GW2.
They existed at the time of Orr, since there were undead horsemen in GW1. The imagery of knights on horseback persists even to the present day Tyria (check out the pictures of the festival in DR, for example), and two of the guild symbols it is available is that of unicorns.
Pictures depicting horses and horselike creatures from the wiki:
http://wiki.guildwars2.com/images/thumb/5…concept_art.jpg
http://wiki.guildwars2.com/images/0/09/Guild_emblem_48.png
http://wiki.guildwars2.com/images/8/8b/Guild_emblem_49.png
There are also horses mentioned in Edge of Destiny, page 97:
In Edge of Destiny a herd of wild horses is seen through an asura gate when Logan, Rytlock, and Caithe flee from the Destroyer horde in the Dwarven ruins.
“Visions. Beautiful visions… a grassy plain where wild horses ran… a deep lagoon encircled by leaning palms…”
There’s horses in the concept art:
http://wiki.guildwars2.com/images/thumb/4…concept_art.jpg
This lore bit here also mentions skilled horsemen:
An Empire Divided - Guild Wars Wiki (GWW)
And here’s a dude in original GW that mentions riding horses:
Zumo the Beggar - GuildWars Fanfic Wikia, an unofficial Guild Wars Fan Fiction wiki
Hey, hey you! I was riding my horse to the other side of town and I ran out of hey to feed him to get there. Spare a few gold so I can feed him and be on my way?
So there are definitely horses in the lore, all over the place.
The better question is, why don’t we SEE any horses in Tyria?