How do I get Mythical Cavalry as Skewered Drake?

As the title says. For one of the first missions in the Skewered Drake MT I need to have 4 regiments of mythical cavalry. The Problem is, that I can't recruit any. I know from past posts, that they are a type of mercenary unit without the DLC. But the unit just doesn't show up in the merc tab, even after hundreds of years. Do I need to fullfill a certain condition that I don't know about, or is my game just bugged? I play the bitbucket version and updated to the most recent one just in case.