Getting your first mount

[Garlyle]

I think from a technical position, all the mounts are programmed to run at the same speed. However, due to individual aspects of certain mounts, some seem to run slightly slower or faster than others (this is most noticible with mounts that are low to the ground, like the cavalry drake and coruel - because they're closer to the ground it seems they're faster. However, the taller ones like the goobbue and magitek armour seem to be much slower due to their size.

This is correct. All the mounts move at the same speed - some will just look faster/slower because their model is larger, but they're covering the same amount of distance in the same amount of time.

Your choice of mounts is purely cosmetic.