"Modern" here seems to be pretty broad and consists largely of works made when horse was the main mode of transport. And in fact, works after Muybridge in the 1

"Modern" here seems to be pretty broad and consists largely of works made when horse was the main mode of transport. And in fact, works after Muybridge in the 1880s are more accurate than those before. So I don't know if lack of access to quadrupeds would explain it. On the other hand have we controlled for quadruped type? At a guess, later artworks are largely of horses, and horses' legs are thin and move fast. Mammoths' legs are huge and move slowly, making them easier to observe. Horses also have multiple gaits, and the faster ones are very difficult to observe.