Interestingly, a quick search of the name "Mongol Khan" also leads to the February 1987 NES game Flying Dragon: The Secret Scroll (the sprite in this set matches the Mongol Khan listing here). While the name 'Mongol Khan' is about par for the course in terms of Punch Out cultural stereotypes, I wonder if they were perhaps originally planning a crossover?

Alternatively, they could have found out that the name wasn't unique and chose an alternate character idea instead.