Why single out mounts? Gear scales too, you don't find it odd that a Galka can pass a breastplate to a Taru but it magically fits both of them? At least mounts can be explained away, lore-wise (ie different breeds, younger mounts etc). If this really bugs you, why doesn't gear scaling?

Also, with the exception of the PVP warhorses there aren't any horses in the game. Sleipnir is a magical creature and the Unicorns are unicorns.

Edit: I don't know why we're using XI terms, but I like it