Peaked about A.D. 1200. Caused by the Mongols invading Central Asia, Eastern Europe, China and the Middle East in their SUVs. The medieval chroniclers didn’t mention the SUVs because there was no word in Latin for them.
Dat’s right... And all dat Mongol pony crap suddenly apearing in Europe, hunnert thousands of ponies all crapping from the steppes all de way.
To dis very day, dung bettles scour the old Mongol trails looking for crap mounds.