As far as the "axle" issue goes, I don't see any particular connection to horses, wagons and reins. The axle(s) of a wagon would be nowhere near the reins, even if you dropped them. Besides, most harnesses have keepers to prevent the reins from dropping down. I think it's just a generic axle, as any axle (or any spinning thing, for that matter) is going to get messed up if something wraps around it.

From Wiki, Bran: "It gets its common name from the legend that other smaller species of fish could ride on the back of it over great distances."

:0)