Riding Crop and similar items have been in limbo since the beginning of Wrath's beta (maybe even the alpha). Originally, those items were turned into enchantments that are applied to a single mount so you didn't need to worry about them in your trinket slot anymore. Then mounts became abilities that you learn rather than items that sit in your bags, to free up bag space. This broke the new implementation of the Riding Crops, and Blizzard has been trying to decide what to do with them since then.
They've decided. Riding Crops, Carrots on a Stick, Skybreaker Whip, all of that jazz? Gone. Done. No more. They're being removed entirely. (Edit: Okay, they're gone after level 70. So they're mostly gone. Riding Crop has a level 69 requirement, you get to use it for about 2 levels.) I hope you had fun farming up those Primal Mights to make Crops for all of your alts. I also hope you didn't want to keep them, because you're out of luck!
Spells/abilities like Crusader Aura and On a Pale Horse will still work, but items will not do anything. Ghostcrawler says they have plans for new implementations of improving mounted speed, and my money is on sucking another 1,000g out of you for a flat 10% speed increase. After all, that's about what it costs to fly in chilly climates, apparently.