Adding Mounts to NPCs
 

I am trying to create the Drake Riders from Wrath of the Righteous. They have Rift Drakes as mounts. I can pull Rift Drakes from the vault, but do not see a way to tie them to the Drake Riders. Mounts go on their riders initiative. Consequently, I need to be able to tie them together on the "stage" so that they appear next to each other in the initiative stack.

I can see how to add a few different mounts (horses, etc.) on the "Companions and Transportation" area under "Equipment>Gear." There is also a spot to add minions under "Equipment>Gear>Purchasable Minions." I do not see an option to add a Rift Drake or custom mount/minion under either.

How do I set up the Drake Riders to reflect their Rift Drake mounts?