Royal Janissary is an untrainable gunpowder unit in . It is only available in the Age of Empires II HD: The Forgotten Scenario Editor. Apart from the HP, it has exactly the same stats as the Elite version of the regular Turkish unique unit, the Janissary.
The regular Janissary was originally planned to have the design used by the Royal variant, the reason for the change was "Because testers and Ensemble employees would comment that the Janissary looked like a "Ku Klux Klan" member".
Despite not featuring as a unit until The Forgotten, Royal Janissaries are shown stationed besides a Mosque in one of the screenshots that play at the start of the Marching and Fighting scenario in the William Wallace Learning Campaign in The Age of Kings.
A regular Janissary (on the left) and a Royal Janissary
A group of Royal Janissaries
A group of Royal Janissaries as seen in the opening video of the William Wallace Learning Campaign
ROYAL JANISSARY vs EVERY UNIQUE UNIT AoE II Definitive Edition