Pony Up!

Here's some more details.

It takes 150 Champion's Seals to buy it and you have to be eligible, I believe it's Exalted Argent Champion of the Alliance/Exalted Argent Champion of the Horde that's required? Anyway.

You get [item=47541], a consumable, and you use it, and it disappears, giving you Pony Up!. You retain the original Argent Squire/Argent Gruntling pet spell you had before, however additional options become available when you talk with your Squire/Gruntling; to bank, vendor, or mailbox. Should you actually use any of these options, he gets a 3 minute buff before running off. It's then 4 hours before the options appear again even if you resummon him in the meantime.

If, like me, all you want to do is just see him ride his little pony, just have him out while YOU'RE mounted. He'll follow on his pony.

While activating this mode for your Squire/Gruntling unlocks an achievement of its own, it will not count towards any other achievements for mounts or pets.

Don't be a twit like me and purchase the Argent Horse for 100 Champion's Seal because it has the same thumbnail....

It is a shame that this achievement is not called "For Pony!"