“Genghis Khan must have needed a lot of horses that produced a lot of carbon, especially running around a lot.”
I just finished a 1928 tome on Ghengis Khan and the Mongol conquests. It’s estimated that his armies were in the range of 300,000 in size total. Each warrior had a string of ponies. In the distance runs they could travel 100 miles/day as a unit by changing horses.
It’s not inconceivable that in the first few years of conquest the Mongols went through a few million ponies.
More recent scholarship has found that his armies were really much smaller.
Basically, in every country where the Mongols showed up and kicked ass, their historians explained it away by claiming to be outnumbered - hence the term "horde" becoming synomous with massive numbers.
In reality, the Mongols were pretty badly outnumbered in pretty much every battle they ever fought, and won through superior discipline and tactical skill.