Preface - This is somewhat useless except for the cool feeling you have when you're sitting amongst a group of randoms in Dalaran toting a giant gargoyle or skeletal dragon...
You must be a DK with access to the spell Control Undead. You must not have a ghoul out.
Steps: Go to Icecrown Citadel and find one of the Citadel Watcher gargoyles posted along the ledges around the large stairway descending from the entrance of ICC, or the better option IMO is the Frostbrood Skytalon (the same model as the Bloodbathed Frostbrood Vanquisher mount from the Glory of the Icecrown Raider (10) meta achievement. For option 2, just aggro the dragon by flying through it a few times until you start getting attacked and then SLOWLY descend towards the ground or to a closeby ledge, dismount, and use Control Undead on it. Since these mobs can fly, they will in fact follow you when you mount up and fly away. Just in case you have max flying speed, make sure you don't outrun them (I haven't leashed one accidentally yet but safety first!). Fly into Dalaran and display your shiny new minion! Unfortunately, when the Control spell wears off after 5 mins, the mob will just leash back to Icecrown so no mass killing the citizens of Dalaran with a fleet of undead minions but I think it's still a ton of fun seeing everyone's reactions. As an added fun tip, use the "Move to" command on your pet hotbar and have the dragon or gargoyle fly through groups of people without you to make it more odd for the onlookers!
I do apologize that this isn't anything uber-1337 but I've been lurking on these boards for years and just wanted to contribute something at least. I do hope somebody enjoys this guide! Happy hunting =D
EDIT ALERT: I've found this MUCH more fun when using a Frostbrood Matriarch. It's similar in model to the Skytalon except about triple the size. They can be found circling close to the ground in Icecrown @ 46.00, 63.00. I had one inside the AH in Dal today and people were pretty damn confused to say the least.