The term that best describes what is happening is coevolution. This concept is used to appoint the mutal evolutionary adapatation of two species. In this case, the prey species has had a selection pressure due to predator species, where the fastest running individuals were able to scape from predators, survive and therefore produce a faster offspring. On the side of predators, there has been also a selection pressure which has filtered out slow individuals too, this time because a greater running speed increases the chances of capturing prey and thus surviving and reproducing.